Sql Interview Questions For Freshers & Experienced 2024
Sql Interview Questions For Freshers
1. What is SQL?
SQL stands for Structured Query Language. It is used to communicate with databases to perform various operations like creating, modifying, deleting, and retrieving data. SQL is an industry standard and helps in managing data efficiently within a database. It supports various commands like SELECT, INSERT, UPDATE, and DELETE, which are used to manipulate the data.
2. What is a Database?
A database is a structured collection of data that is stored and managed electronically in a computer system. It consists of tables, schemas, queries, and views that help in organizing and storing data systematically. Databases are used to store large amounts of information and allow easy access and manipulation using SQL.
3. What is the difference between SQL and PL/SQL?
SQL is a declarative query language that is used for performing operations like inserting, updating, and deleting records in a database. On the other hand, PL/SQL (Procedural Language/SQL) is an extension of SQL used in Oracle databases that adds procedural features such as conditions, loops, and error handling, allowing for more complex operations.
4. Write a query to find the names of employees starting with ‘A’.
This query uses the LIKE operator to search for names that start with the letter ‘A’:
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SELECT * FROM Employees WHERE EmpName LIKE ‘A%’;
The % symbol acts as a wildcard, indicating that any number of characters can follow after ‘A’. This query will return all employees whose names begin with ‘A’.
5. What is the difference between CHAR and VARCHAR datatypes?
CHAR: This datatype is used to store fixed-length character strings. For example, if a CHAR column is defined as CHAR(10), it will always store 10 characters, padding with spaces if necessary.
VARCHAR: This datatype is used to store variable-length character strings. For example, if a VARCHAR column is defined as VARCHAR(10), it will store only the entered characters without any additional padding.
6. What is a Foreign Key?
A foreign key is a field (or a collection of fields) in one table that uniquely identifies a row of another table. It is used to create a relationship between the two tables, enforcing referential integrity by ensuring that data in one table corresponds to valid entries in another table. For example, if Table A has a foreign key column that references Table B, all values in this column must exist in Table B.
7. Explain the types of SQL Joins.
SQL Joins are used to combine records from two or more tables based on a related column between them:
INNER JOIN: Returns rows that have matching values in both tables.
LEFT JOIN: Returns all rows from the left table and the matched rows from the right table. Non-matching rows from the right table will have NULL values.
RIGHT JOIN: Returns all rows from the right table and the matched rows from the left table. Non-matching rows from the left table will have NULL values.
FULL JOIN: Combines the results of both LEFT JOIN and RIGHT JOIN and returns all rows when there is a match in either left or right table.
8. What is Normalization?
Normalization is a technique used in database design to organize data to reduce redundancy and dependency. It involves dividing a database into two or more tables and defining relationships between them to ensure that data is stored efficiently. The main purpose is to minimize data duplication and maintain data integrity.
9. What is a View in SQL?
A view is a virtual table that is based on the result of an SQL query. It allows you to store complex queries in a structured format and can be used to simplify data retrieval by encapsulating complex joins and subqueries. Views do not store data physically; instead, they provide a way to look at the data stored in other tables.
10. What is a Subquery?
A subquery is a query within another SQL query, nested inside a SELECT, INSERT, UPDATE, or DELETE statement. It is used to return data that will be used in the main query as a condition. Subqueries are enclosed in parentheses and can be used to filter results or generate computed columns.
11. What is the difference between DELETE and TRUNCATE commands?
DELETE: Deletes rows from a table based on a condition and records each deletion in the transaction log. It is slower as it removes rows one at a time and can be rolled back if needed.
TRUNCATE: Deletes all rows from a table by deallocating the data pages, which is much faster than DELETE. It cannot be rolled back and resets any identity columns.
12. What is a Stored Procedure?
A stored procedure is a set of SQL statements stored in the database that can be executed as a single unit. It allows you to encapsulate logic and perform complex operations like conditional statements and loops. Stored procedures help in reducing network traffic and improve performance by executing multiple SQL statements in one go.
13. What are Aggregate Functions in SQL?
Aggregate functions perform calculations on a set of values and return a single value. Examples include:
COUNT(): Counts the number of rows.
SUM(): Calculates the sum of a numeric column.
AVG(): Calculates the average of a numeric column.
MAX(): Returns the maximum value.
MIN(): Returns the minimum value.
14. What is SQL Injection?
SQL injection is a type of security vulnerability where an attacker injects malicious SQL code into a query to manipulate the database. This can lead to unauthorized access, data manipulation, or data leakage. It is usually done by inserting SQL commands into input fields of web forms, making it important to use parameterized queries to prevent such attacks.
15. What is the difference between UNION and UNION ALL?
UNION: Combines the results of two queries and removes duplicate rows.
UNION ALL: Combines the results of two queries without removing duplicates, making it faster as it does not check for duplicates.
16. What are ACID properties?
ACID stands for:
Atomicity: Ensures that all operations within a transaction are completed successfully.
Consistency: Ensures that the database remains in a consistent state before and after a transaction.
Isolation: Ensures that multiple transactions do not interfere with each other.
Durability: Ensures that the results of a committed transaction are permanently saved in the database, even if a system failure occurs.
17. What is a Primary Key?
A primary key is a unique identifier for each record in a table. It ensures that no duplicate values are stored in the primary key column, and each row can be uniquely identified. A table can have only one primary key, which may consist of single or multiple columns.
18. What is the difference between DROP and TRUNCATE statements?
DROP: Removes a table and its structure from the database, along with any associated indexes, constraints, and permissions.
TRUNCATE: Deletes all rows from a table but keeps its structure intact, allowing new data to be inserted into it later.
19. What is a Trigger?
A trigger is a set of SQL commands that automatically executes in response to certain events in a table, such as an INSERT, UPDATE, or DELETE operation. Triggers are used to enforce data integrity, automate tasks, and maintain logs of changes in the database.
20. What is an Index in SQL?
An index is a database object that helps improve the speed of data retrieval operations. It works like a pointer to the data in a table and allows faster access to rows based on the indexed columns. Indexes can be created on one or more columns and are especially useful for large tables.
21. What are the different types of SQL Constraints?
SQL constraints are rules enforced on table columns to maintain data integrity. Some of the commonly used constraints are:
NOT NULL: Ensures that a column cannot have a NULL value.
UNIQUE: Ensures all values in a column are unique.
PRIMARY KEY: Uniquely identifies each row in a table.
FOREIGN KEY: Ensures referential integrity by linking a column to the primary key of another table.
CHECK: Ensures that the value in a column meets a specific condition.
DEFAULT: Sets a default value for a column when no value is specified.
22. What is a Composite Key?
A composite key is a combination of two or more columns in a table that, together, uniquely identify a row. It is used when no single column can uniquely identify a row. For example, in a Student table, combining the StudentID and CourseID columns can uniquely identify each student’s course enrollment.
23. What is the difference between HAVING and WHERE clause?
WHERE: Filters rows before they are grouped and does not work with aggregate functions.
HAVING: Filters rows after grouping and can use aggregate functions like SUM(), COUNT(), etc. The HAVING clause is typically used with GROUP BY.
24. What is a Cursor in SQL?
A cursor is a database object used to retrieve and manipulate a set of rows one at a time. It is primarily used in stored procedures, triggers, or batch scripts for iterative row-by-row operations. Cursors can be declared, opened, fetched, and closed during their life cycle.
25. What is a Self-Join in SQL?
A self-join is a join where a table is joined with itself. This is used when a table has a relationship with itself, such as an employee table where each employee has a ManagerID that references another employee in the same table.
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SELECT A.EmployeeName, B.EmployeeName AS ManagerName
FROM Employees A, Employees B
WHERE A.ManagerID = B.EmployeeID;
26. What is a Cross Join?
A cross join returns the Cartesian product of two tables, meaning it returns all combinations of rows from both tables. If table A has 3 rows and table B has 2 rows, a cross join will return 6 rows (3×2). It is generally used for generating combinations and rarely used in practical applications due to its large result set.
27. What are Group Functions in SQL?
Group functions (or aggregate functions) operate on multiple rows and return a single result value. These include:
COUNT(): Returns the number of rows.
SUM(): Returns the sum of a numeric column.
AVG(): Returns the average value.
MIN(): Returns the minimum value.
MAX(): Returns the maximum value.
28. What is the difference between a Local and a Global Variable in SQL?
Local Variable: Declared and used within a function or procedure. Its scope is limited to the function where it is defined.
Global Variable: Declared outside of any function and can be accessed by any part of the program. It has a global scope and exists throughout the session.
29. What is the use of the NVL() function in SQL?
The NVL() function in SQL is used to replace NULL values with a specified value. It is often used to ensure that a column value does not remain NULL in computations or query results. For example:
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SELECT NVL(commission, 0) FROM Employees;
This query replaces any NULL values in the commission column with 0.
30. What is an Alias in SQL?
An alias is a temporary name given to a table or a column for the purpose of making the query more readable and easier to understand. Aliases are used with the AS keyword. For example: sql
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SELECT EmployeeID AS ID, EmployeeName AS Name FROM Employees;
Here, EmployeeID is temporarily renamed as ID and EmployeeName as Name in the result set.
31. What is a Temporary Table in SQL?
A temporary table is a table that is created and exists temporarily during a database session or until the program ends. It is useful for storing intermediate results or performing operations on data without affecting the original table. Temporary tables are created using the CREATE TEMPORARY TABLE statement.
32. What is SQL Profiler?
SQL Profiler is a tool used to monitor and analyze SQL Server events, such as SQL queries, stored procedures, and user activity. It helps in identifying performance issues, tracking slow-running queries, and debugging database problems. SQL Profiler provides insights into how SQL statements interact with the database.
33. What is an Execution Plan in SQL?
An execution plan is a visual representation of how the SQL Server executes a query. It shows the steps taken to retrieve data and includes information on how tables are accessed, joins are performed, and indexes are used. Execution plans are used to optimize query performance by identifying bottlenecks.
34. What is a Clustered Index?
A clustered index determines the physical order of data in a table. There can be only one clustered index per table because the rows are stored on the disk in the same order as the clustered index. Clustered indexes are used for fast retrieval of data and efficient range-based queries.
35. What is a Non-Clustered Index?
A non-clustered index creates a logical order for data rows and stores pointers to the physical data in the table. A table can have multiple non-clustered indexes. Non-clustered indexes are useful for columns frequently used in search criteria or join conditions to speed up data retrieval.
36. What is a Data Integrity Constraint?
Data integrity constraints are rules applied to table columns to ensure the accuracy and consistency of data. Common constraints include:
NOT NULL: Prevents null values in a column.
UNIQUE: Ensures that each value in a column is unique.
CHECK: Ensures that all values in a column satisfy a specific condition.
PRIMARY KEY: Uniquely identifies each record in a table.
FOREIGN KEY: Maintains referential integrity between tables.
37. What is an ETL Process?
ETL stands for Extract, Transform, and Load. It is a process used in data warehousing to extract data from different sources, transform it into a required format, and then load it into a target data warehouse. ETL is essential for consolidating data for analysis and reporting.
38. What are Scalar Functions in SQL?
Scalar functions return a single value based on input values. Examples include:
UPPER(): Converts a string to uppercase.
LOWER(): Converts a string to lowercase.
LENGTH(): Returns the length of a string.
ABS(): Returns the absolute value of a number.
39. What is the COALESCE() function in SQL?
The COALESCE() function returns the first non-null value from a list of arguments. It is used to handle NULL values in expressions and can work with multiple columns or values. For example: sql
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SELECT COALESCE(column1, column2, ‘No Value’) FROM table_name;
If column1 and column2 are both NULL, it will return ‘No Value’.
40. What is a Case Statement in SQL?
The CASE statement is used to provide conditional logic in SQL queries, similar to an IF-THEN-ELSE statement. It allows you to specify conditions and return values based on whether these conditions are met. For example: sql
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SELECT EmployeeID,
CASE
WHEN Salary > 50000 THEN ‘High Salary’
ELSE ‘Low Salary’
END AS SalaryCategory
FROM Employees;
This query categorizes employees based on their salary.
Sql Interview Questions For Freshers
41. What is a Transaction in SQL?
A transaction is a sequence of SQL operations that are treated as a single unit of work. Transactions are used to maintain data integrity by ensuring that all operations are completed successfully. If any operation fails, the entire transaction is rolled back. Transactions use commands like BEGIN, COMMIT, and ROLLBACK.
42. What is SQL Injection, and how can it be prevented?
SQL injection is a security vulnerability where malicious SQL code is injected into a query to manipulate the database. It can lead to unauthorized access, data leakage, and database corruption. To prevent SQL injection, use parameterized queries, stored procedures, and input validation.
43. What is a Union in SQL?
The UNION operator combines the results of two or more SELECT statements and removes duplicate rows. All SELECT statements must have the same number of columns and compatible data types. For example: sql
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SELECT FirstName FROM Students
UNION
SELECT FirstName FROM Employees;
44. What is a Full Outer Join?
A FULL OUTER JOIN returns all rows from both tables when there is a match between the columns in either table. Rows that do not have a match in the other table are displayed with NULL values.
45. What is a Nested Trigger?
A nested trigger is a trigger that performs an operation that fires another trigger. For example, if an INSERT trigger on Table A performs an INSERT on Table B, which has its own INSERT trigger, the INSERT on Table B will fire the second trigger.
46. What is the difference between ISNULL() and COALESCE()?
ISNULL(): Takes only two arguments and replaces a NULL value with a specified value.
COALESCE(): Takes multiple arguments and returns the first non-null value.
47. What is the purpose of the DISTINCT keyword?
The DISTINCT keyword is used in SELECT statements to return only unique rows. It removes duplicate rows from the result set. For example: sql
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SELECT DISTINCT City FROM Customers;
This query will return unique cities from the Customers table.
48. What is a Schema in SQL?
A schema is a collection of database objects like tables, views, and procedures that are grouped together. It helps in organizing the database and controls access to these objects. Schemas provide a way to logically separate database objects.
49. What are the SET Operations in SQL?
SET operations in SQL include UNION, UNION ALL, INTERSECT, and EXCEPT. These operations combine results from two or more queries into a single result set. Each operation has different rules for handling duplicates and matching rows.
50. What are Aggregate Functions?
Aggregate functions perform calculations on multiple rows of data and return a single value. Common aggregate functions include:
COUNT(): Counts the number of rows.
SUM(): Calculates the total sum of a column.
AVG(): Calculates the average value.
MIN(): Finds the minimum value.
MAX(): Finds the maximum value.
Sql Interview Questions For Experienced
51. What is a Materialized View?
A materialized view is a database object that contains the results of a query. Unlike a regular view, which only stores the query logic, a materialized view stores the actual data and can be refreshed periodically. It is useful for improving query performance, especially for complex queries on large datasets. Materialized views are frequently used in data warehousing to store aggregated data.
52. How do you optimize an SQL query?
Query optimization involves several techniques, including:
Using proper indexing: Creating indexes on columns frequently used in WHERE clauses, JOINs, or ORDER BY.
**Avoiding SELECT ***: Selecting only the necessary columns instead of all columns.
Using joins wisely: Replacing subqueries with joins when appropriate.
Avoiding functions on indexed columns: Using functions on indexed columns can lead to a full table scan.
Using query execution plans: Reviewing the execution plan to identify performance bottlenecks.
53. What is the purpose of the SQL Server Profiler?
SQL Server Profiler is a tool that helps in monitoring and analyzing the performance of SQL queries and database activities. It captures events such as SQL queries, stored procedure executions, and user logins. Profiler is useful for identifying long-running queries, deadlocks, and other performance issues in SQL Server.
54. What is the difference between Clustered and Non-Clustered Indexes?
Clustered Index: Determines the physical order of data in a table. There can be only one clustered index per table because the data rows are stored on the disk in the same order as the index.
Non-Clustered Index: Creates a separate structure that points to the location of data in the table. A table can have multiple non-clustered indexes. Non-clustered indexes are ideal for frequently searched columns.
55. What are SQL Server Deadlocks, and how can you resolve them?
A deadlock occurs when two or more transactions are waiting for each other to release a lock on resources, causing both transactions to be stuck. To resolve deadlocks:
Use the TRY…CATCH block: Implement error handling and reattempt the transaction.
Reduce transaction time: Keep transactions short to minimize locking issues.
Use the NOLOCK hint: For read-only operations, use the WITH (NOLOCK) hint to avoid locking.
Break down complex queries: Simplifying large queries can reduce the chances of deadlocks.
56. What are SQL Hints, and when would you use them?
SQL hints are directives used to alter the behavior of the query execution plan. They provide optimization suggestions to the SQL engine. Common hints include:
NOLOCK: Avoids placing locks during data retrieval.
FORCE INDEX: Forces the query to use a specific index.
QUERYTRACEON: Enables or disables specific trace flags that control the behavior of SQL Server.
Hints are used when the SQL optimizer does not choose the best execution plan for a query, and manual intervention is needed.
57. How do you implement pagination in SQL?
Pagination is used to divide query results into smaller subsets (pages). This is commonly done using the OFFSET and FETCH clauses in SQL Server or MySQL:
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SELECT *
FROM Employees
ORDER BY EmployeeID
OFFSET 10 ROWS FETCH NEXT 10 ROWS ONLY;
This query skips the first 10 rows and fetches the next 10 rows. In Oracle, the ROWNUM pseudo column or ROW_NUMBER() function can be used.
58. Explain the concept of Table Partitioning?
Table partitioning is a database optimization technique that divides a large table into smaller, more manageable pieces, called partitions. Each partition can be accessed and managed independently, making queries faster. Partitioning can be done by range, list, hash, or composite methods. It improves query performance and simplifies maintenance of large tables.
59. What is the difference between OLTP and OLAP databases?
OLTP (Online Transaction Processing): Used for managing transactional data, OLTP databases are optimized for insert, update, and delete operations. They support high-volume and short transactions, such as order entry and customer management.
OLAP (Online Analytical Processing): Used for data analysis and reporting, OLAP databases are optimized for complex queries, aggregations, and multi-dimensional analysis. They are typically used in data warehouses for business intelligence purposes.
60. How do you handle NULL values in SQL?
Handling NULL values is crucial to avoid errors or incorrect results. Some techniques include:
COALESCE(): Returns the first non-NULL value among its arguments.
ISNULL(): Replaces NULL with a specified value (SQL Server).
NVL(): Replaces NULL with a specified value (Oracle).
NULLIF(): Returns NULL if the two arguments are equal; otherwise, it returns the first argument.
61. What is the difference between a Temporary Table and a Table Variable in SQL Server?
Temporary Table: Created using CREATE TABLE #TempTable. It supports indexes and transactions and is stored in the tempdb database. Temporary tables are useful for large datasets and complex queries.
Table Variable: Created using the DECLARE @TableVariable syntax. It has a limited scope and is stored in memory, making it faster for small datasets but less flexible than temporary tables.
62. How do you perform error handling in SQL?
Error handling in SQL is performed using the TRY…CATCH block. The TRY block contains the code to execute, while the CATCH block handles any errors that occur.
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BEGIN TRY
— SQL statements that might cause an error
INSERT INTO Employees VALUES (NULL, ‘John’);
END TRY
BEGIN CATCH
— Error handling statements
PRINT ‘An error occurred’;
END CATCH;
This structure helps handle exceptions, roll back transactions, and log errors.
63. What are Common Table Expressions (CTEs)?
CTEs are temporary result sets that can be referenced within the main query. They simplify complex queries and recursive operations. CTEs are defined using the WITH keyword and are often used for hierarchical data.
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WITH EmployeeCTE AS (
SELECT EmployeeID, ManagerID, EmployeeName
FROM Employees
)
SELECT * FROM EmployeeCTE;
CTEs make the code more readable and maintainable.
64. What is the use of the RANK(), DENSE_RANK(), and ROW_NUMBER() functions?
These functions assign a unique ranking to rows within a result set:
RANK(): Assigns a rank to each row, skipping ranks for duplicates.
DENSE_RANK(): Assigns a rank without gaps in the ranking for duplicates.
ROW_NUMBER(): Assigns a unique sequential number to each row.
They are commonly used for pagination and ranking data.
65. What are Window Functions in SQL?
Window functions perform calculations across a set of table rows related to the current row. Unlike aggregate functions, they do not collapse rows into a single output. Examples include:
SUM() OVER(): Computes the cumulative sum.
ROW_NUMBER() OVER(): Assigns a sequential row number.
LEAD() and LAG(): Access the next or previous row’s data.
Window functions allow for advanced analysis within partitions of a result set.
66. What is Database Sharding?
Database sharding is a technique of horizontally partitioning data across multiple databases or servers. It is used to improve scalability and performance by dividing a large dataset into smaller chunks (shards). Each shard contains a subset of the data, and queries are routed to the appropriate shard based on the sharding key.
67. How do you implement recursive queries in SQL?
Recursive queries are implemented using a recursive CTE (Common Table Expression). It involves a CTE that references itself:
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WITH RecursiveCTE AS (
SELECT EmployeeID, ManagerID, EmployeeName
FROM Employees
WHERE ManagerID IS NULL
UNION ALL
SELECT e.EmployeeID, e.ManagerID, e.EmployeeName
FROM Employees e
INNER JOIN RecursiveCTE r ON e.ManagerID = r.EmployeeID
)
SELECT * FROM RecursiveCTE;
This query retrieves hierarchical data, such as an employee reporting structure.
68. What is SQL Injection, and how can it be prevented?
SQL injection is a code injection technique where malicious SQL code is inserted into an input field, allowing attackers to manipulate the database. To prevent SQL injection:
Use parameterized queries or prepared statements.
Validate and sanitize user inputs.
Implement proper error handling to avoid exposing sensitive information.
69. What are the different Isolation Levels in SQL?
Isolation levels determine how transaction integrity is visible to other transactions and ensure data consistency:
Read Uncommitted: Allows dirty reads (uncommitted changes).
Read Committed: Prevents dirty reads but allows non-repeatable reads.
Repeatable Read: Prevents dirty and non-repeatable reads but allows phantom reads.
Serializable: Prevents dirty, non-repeatable, and phantom reads, ensuring the highest level of isolation.
70. What is the use of the STUFF() function in SQL Server?
The STUFF() function deletes a specified length of characters from a string and inserts another set of characters starting at a specified position.
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SELECT STUFF(‘Hello World’, 6, 5, ‘SQL’);
81. What are the differences between UNION and INTERSECT?
UNION: Combines the results of two queries and removes duplicate rows. It returns all unique rows from both queries.
INTERSECT: Returns only the rows that are present in both result sets. It finds common records between two queries.
82. What is a Recursive CTE?
A recursive Common Table Expression (CTE) is a CTE that references itself, allowing for hierarchical or recursive data retrieval. It consists of two parts: the anchor member (the base case) and the recursive member (which references the CTE itself).
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WITH RecursiveCTE AS (
SELECT EmployeeID, ManagerID, EmployeeName
FROM Employees
WHERE ManagerID IS NULL
UNION ALL
SELECT e.EmployeeID, e.ManagerID, e.EmployeeName
FROM Employees e
INNER JOIN RecursiveCTE r ON e.ManagerID = r.EmployeeID
)
SELECT * FROM RecursiveCTE;
This retrieves the entire employee hierarchy.
83. What is Database Normalization?
Database normalization is the process of organizing data to reduce redundancy and improve data integrity. It involves dividing a database into tables and establishing relationships to minimize data duplication. Normal forms (1NF, 2NF, 3NF, etc.) provide guidelines for achieving normalization, helping to ensure that data dependencies are properly enforced.
84. What is a Trigger in SQL?
A trigger is a set of SQL statements that automatically executes in response to specific events on a table, such as INSERT, UPDATE, or DELETE operations. Triggers are used for maintaining data integrity, enforcing business rules, and automatically generating audit logs.
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CREATE TRIGGER trg_Audit
AFTER INSERT ON Employees
FOR EACH ROW
BEGIN
INSERT INTO AuditTable (EmployeeID, Action) VALUES (NEW.EmployeeID, ‘INSERT’);
END;
85. What is the SQL command to create an Index?
An index is created using the CREATE INDEX command. Indexes improve query performance by providing faster data access. The basic syntax is:
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CREATE INDEX index_name ON table_name (column1, column2);
For example:
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CREATE INDEX idx_EmployeeName ON Employees (EmployeeName);
This creates an index on the EmployeeName column.
86. How can you retrieve the last inserted ID in SQL?
To retrieve the last inserted ID, you can use the LAST_INSERT_ID() function in MySQL or the SCOPE_IDENTITY() function in SQL Server. This is useful for obtaining the primary key value of the last inserted row.
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— MySQL
INSERT INTO Employees (Name) VALUES (‘John’);
SELECT LAST_INSERT_ID();
— SQL Server
INSERT INTO Employees (Name) VALUES (‘John’);
SELECT SCOPE_IDENTITY();
87. What is a Cross Join?
A cross join returns the Cartesian product of two tables, meaning it returns every combination of rows from both tables. For example, if Table A has 3 rows and Table B has 2 rows, a cross join will return 6 rows (3×2). Cross joins are rarely used in practice due to the large result sets they can generate.
88. What is the difference between a Stored Procedure and a Function?
Stored Procedure: A stored procedure is a set of SQL statements that can perform operations such as INSERT, UPDATE, DELETE, and SELECT. It can return multiple values or result sets, and it does not necessarily return a value.
Function: A function is a set of SQL statements that performs a specific task and returns a single value. Functions cannot perform actions like modifying the database or generating output parameters.
89. What is SQL Server Agent?
SQL Server Agent is a component of SQL Server that allows for scheduling and executing jobs, such as running scripts, backing up databases, and performing maintenance tasks automatically. It provides alerts and notifications for specific events and is essential for automating routine database administration tasks.
90. What is a Data Warehouse?
A data warehouse is a centralized repository that stores integrated data from multiple sources, optimized for reporting and analysis. It supports business intelligence activities, such as data mining and online analytical processing (OLAP). Data warehouses typically use a star or snowflake schema for organizing data.
91. What is the purpose of SQL Server Integration Services (SSIS)?
SQL Server Integration Services (SSIS) is a powerful data integration and workflow tool that is part of the SQL Server suite. It is used for extracting, transforming, and loading (ETL) data from various sources into a data warehouse or database. SSIS allows you to automate data movement and transformation tasks, making it an essential tool for data warehousing and business intelligence projects.
92. What is an execution plan, and how do you analyze it?
An execution plan is a graphical representation of the steps SQL Server takes to execute a query. It shows how tables are accessed, which indexes are used, and the order of operations. You can analyze the execution plan to identify performance issues, such as missing indexes or inefficient joins. In SQL Server Management Studio (SSMS), you can view the execution plan by including the “Include Actual Execution Plan” option before running a query.
93. How can you implement data encryption in SQL Server?
Data encryption in SQL Server can be implemented using:
Transparent Data Encryption (TDE): Protects data at rest by encrypting the database files. It requires a database master key and an encryption certificate.
Column-level encryption: Allows you to encrypt specific columns using symmetric or asymmetric keys. Use the ENCRYPTBYKEY() function for encryption and DECRYPTBYKEY() for decryption.
94. What is the difference between UNION and UNION ALL?
UNION: Combines the results of two or more SELECT statements and removes duplicate rows from the result set.
UNION ALL: Combines the results without removing duplicates, returning all rows from both queries. It is generally faster than UNION because it does not require a duplicate check.
95. How do you create a backup of a SQL Server database?
To create a backup of a SQL Server database, you can use the following T-SQL command:
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BACKUP DATABASE database_name TO DISK = ‘C:\Backup\database_name.bak’;
You can also perform backups using SQL Server Management Studio (SSMS) by right-clicking the database, selecting “Tasks,” and then choosing “Back Up.”
96. What is a stored function, and how is it different from a stored procedure?
A stored function is a set of SQL statements that perform a specific task and return a single value. Functions can be used in SQL expressions, while stored procedures cannot. Functions are generally used for calculations, while stored procedures can perform operations like data modification and return multiple result sets.
97. How can you find duplicate records in a SQL table?
To find duplicate records, you can use the GROUP BY clause along with the HAVING clause. For example:
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SELECT column1, COUNT(*) AS duplicate_count
FROM table_name
GROUP BY column1
HAVING COUNT(*) > 1;
This query returns the values in column1 that have duplicates along with their counts.
98. What are Window Functions, and how are they different from regular aggregate functions?
Window functions perform calculations across a set of rows related to the current row, while regular aggregate functions group rows and return a single result for each group. Window functions allow you to access additional rows in the result set without collapsing them. Examples include ROW_NUMBER(), RANK(), and SUM() OVER().
99. What is a database trigger, and when would you use it?
A database trigger is a set of SQL statements that automatically execute in response to specific events on a table, such as INSERT, UPDATE, or DELETE operations. Triggers are used to enforce business rules, maintain audit trails, or automatically update related tables. For example, a trigger can be used to log changes to a table whenever a row is inserted.
100. How do you use the COALESCE function in SQL?
The COALESCE function returns the first non-null value from a list of arguments. It is useful for handling NULL values in queries. For example:
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SELECT COALESCE(column1, column2, ‘Default Value’) AS Result
FROM table_name;
This query will return the value from column1 if it is not NULL; otherwise, it will check column2, and if both are NULL, it returns ‘Default Value’.
101. What are the types of indexes in SQL Server?
The main types of indexes in SQL Server include:
Clustered Index: Determines the physical order of data in a table. Each table can have only one clustered index.
Non-Clustered Index: Creates a separate structure that points to the data in the table. A table can have multiple non-clustered indexes.
Full-Text Index: Used for full-text searches, allowing efficient searching of large text columns.
102. What is the purpose of the GROUP BY clause?
The GROUP BY clause is used to group rows that have the same values in specified columns into summary rows, like totals or averages. It is often used with aggregate functions like SUM(), AVG(), and COUNT(). For example:
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SELECT DepartmentID, COUNT(*) AS EmployeeCount
FROM Employees
GROUP BY DepartmentID;
This query counts the number of employees in each department.
103. How can you manage transaction isolation levels in SQL Server?
Transaction isolation levels control the visibility of changes made by one transaction to other transactions. In SQL Server, you can set the isolation level using the SET TRANSACTION ISOLATION LEVEL statement. The available levels are:
READ UNCOMMITTED
READ COMMITTED
REPEATABLE READ
SNAPSHOT
SERIALIZABLE For example, to set the isolation level to READ COMMITTED:
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SET TRANSACTION ISOLATION LEVEL READ COMMITTED;
104. What is SQL Server Agent, and how is it used?
SQL Server Agent is a component of SQL Server that allows you to automate and schedule tasks, such as running SQL jobs, backing up databases, and executing scripts. It enables database administrators to perform routine maintenance tasks without manual intervention. Jobs can be scheduled to run at specific times or triggered by events.
105. What are aggregate functions, and can you name a few?
Aggregate functions perform calculations on a set of values and return a single result. Common aggregate functions include:
COUNT(): Counts the number of rows.
SUM(): Calculates the total of a numeric column.
AVG(): Calculates the average value of a numeric column.
MAX(): Finds the maximum value in a column.
MIN(): Finds the minimum value in a column.
106. How do you handle errors in SQL Server?
Errors in SQL Server can be handled using TRY…CATCH blocks. This structure allows you to catch exceptions and respond accordingly. You can log errors, perform rollback operations, or return custom error messages. For example:
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BEGIN TRY
— SQL statements that might cause an error
INSERT INTO Employees VALUES (NULL, ‘Jane’);
END TRY
BEGIN CATCH
PRINT ‘An error occurred: ‘ + ERROR_MESSAGE();
END CATCH;
107. What is the difference between a primary key and a unique key?
Primary Key: Uniquely identifies each record in a table and cannot contain NULL values. There can only be one primary key in a table.
Unique Key: Ensures that all values in a column are unique but can contain one NULL value. A table can have multiple unique keys.
108. What is the difference between a Database and a Data Warehouse?
Database: Designed for transactional processing (OLTP), focusing on real-time operations and updates. Databases are optimized for insert, update, and delete operations.
Data Warehouse: Designed for analytical processing (OLAP), focusing on data analysis and reporting. Data warehouses are optimized for read-heavy operations and complex queries over large datasets.
109. What is the purpose of the ANALYZE command in SQL?
The ANALYZE command is used to collect statistics about the distribution of data in a table or index, which helps the SQL optimizer create efficient query plans. Regularly running ANALYZE can improve query performance by ensuring that the optimizer has up-to-date information about data distribution.
110. How can you implement a many-to-many relationship in a database?
To implement a many-to-many relationship, you create a junction table (also known as a bridge or associative table) that includes foreign keys referencing the primary keys of the two related tables. For example, if you have Students and Courses tables, the junction table Enrollments would include StudentID and CourseID as foreign keys to represent the many-to-many relationship.