SQL Language Interview Questions

Checkout Vskills Interview questions with answers in SQL Language to prepare for your next job role. The questions are submitted by professionals to help you to prepare for the Interview.

Q.1 What is SQL?

SQL stands for Structured Query Language, which is a domain-specific language used for managing and manipulating relational databases.

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Q.2 What are the different categories of SQL commands?

SQL commands are categorized into Data Query Language (DQL), Data Definition Language (DDL), Data Manipulation Language (DML), and Data Control Language (DCL).

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Q.3 What is a relational database?

A relational database is a type of database that uses a structured format with tables (relations) to store and manage data, with relationships defined between tables.

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Q.4 What is a table in SQL?

A table in SQL is a structured collection of data organized into rows and columns, where each row represents a record and each column represents a data attribute.

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Q.5 What is a primary key in SQL?

A primary key is a column or set of columns in a table that uniquely identifies each row. It enforces data integrity and ensures that no duplicate rows exist.

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Q.6 What is an SQL query?

An SQL query is a request made to a database using SQL statements to retrieve, manipulate, or modify data stored in the database tables.

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Q.7 What is a SELECT statement in SQL?

The SELECT statement is used to retrieve data from one or more tables in a database. It specifies the columns to retrieve and optional filtering conditions.

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Q.8 What is the syntax for a basic SELECT statement?

sql SELECT column1, column2 FROM table_name WHERE condition; is anexample.

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Q.9 How do you retrieve all records from a table in SQL?

To retrieve all records from a table, you can use the SQL statement: sql SELECT * FROM table_name; command.

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Q.10 What is a WHERE clause in SQL?

The WHERE clause is used in SQL queries to filter records based on specified conditions. It narrows down the results to only those that meet the criteria.

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Q.11 What is the purpose of the ORDER BY clause in SQL?

The ORDER BY clause is used to sort the result set of a query in ascending or descending order based on one or more columns.

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Q.12 What is a JOIN in SQL?

A JOIN in SQL combines rows from two or more tables based on a related column between them. It is used to retrieve data from multiple tables in a single query.

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Q.13 What is an INNER JOIN in SQL?

An INNER JOIN retrieves records that have matching values in both tables being joined, excluding non-matching records from the result set.

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Q.14 What is an OUTER JOIN in SQL?

An OUTER JOIN retrieves records from both tables being joined, including unmatched records from one or both tables, with NULL values for non-matching columns.

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Q.15 What is a subquery in SQL?

A subquery, also known as a nested query, is a query embedded within another query. It can be used to retrieve values to filter or manipulate the outer query's results.

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Q.16 What is the purpose of the GROUP BY clause in SQL?

The GROUP BY clause is used to group rows from a table based on the values in one or more columns. It is often used with aggregate functions to summarize data.

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Q.17 What is an aggregate function in SQL?

An aggregate function is a SQL function that performs a calculation on a set of values and returns a single result. Common aggregate functions include SUM, COUNT, AVG, MAX, and MIN.

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Q.18 What is the HAVING clause in SQL?

The HAVING clause is used in conjunction with the GROUP BY clause to filter grouped rows based on aggregate function results. It acts as a filter for grouped data.

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Q.19 What is a SQL view?

A SQL view is a virtual table created from the result of a SELECT query. It can be queried like a regular table and provides a way to simplify complex queries or restrict data access.

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Q.20 What is a stored procedure in SQL?

A stored procedure is a precompiled collection of one or more SQL statements that can be executed as a single unit. It is often used for code reusability and security.

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Q.21 What is a transaction in SQL?

A transaction in SQL represents a series of one or more SQL statements that are executed as a single unit of work. Transactions ensure data consistency and integrity.

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Q.22 What is the COMMIT statement in SQL?

The COMMIT statement is used to save all the changes made during a transaction to the database permanently. It ends the transaction and makes the changes visible to other users.

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Q.23 What is the ROLLBACK statement in SQL?

The ROLLBACK statement is used to undo all the changes made during a transaction, reverting the database to its previous state. It cancels the transaction.

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Q.24 What is SQL injection?

SQL injection is a security vulnerability where malicious SQL code is injected into input fields, leading to unauthorized access, data theft, or database manipulation.

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Q.25 How can you prevent SQL injection in SQL queries?

To prevent SQL injection, use parameterized queries or prepared statements, validate input, and implement proper user authentication and authorization.

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Q.26 What is a database index in SQL?

A database index is a data structure that improves the speed of data retrieval operations on a database table. It provides a quick way to look up rows based on column values.

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Q.27 What is normalization in SQL?

Normalization is the process of organizing data in a database to eliminate redundancy and improve data integrity. It involves breaking down tables into smaller, related tables.

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Q.28 What is denormalization in SQL?

Denormalization is the opposite of normalization. It involves adding redundancy to a database to improve query performance by reducing the number of JOIN operations.

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Q.29 What is the difference between UNION and UNION ALL in SQL?

UNION combines the result sets of two or more SELECT queries, removing duplicate rows, while UNION ALL includes all rows, including duplicates, in the combined result set.

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Q.30 What is an SQL constraint?

An SQL constraint is a rule defined on a table column to enforce data integrity. Common constraints include PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and NOT NULL.

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Q.31 What is the purpose of the FOREIGN KEY constraint in SQL?

The FOREIGN KEY constraint establishes a link between two tables by enforcing referential integrity. It ensures that values in one table's column match values in another table's column.

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Q.32 What is a self-join in SQL?

A self-join is a type of JOIN where a table is joined with itself. It is often used when data in a single table has a hierarchical relationship or when querying related records.

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Q.33 What is the SQL CASE statement used for?

The SQL CASE statement is used for conditional logic in SQL queries. It allows you to perform different actions based on specified conditions within a single query.

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Q.34 What is the purpose of the SQL INDEX hint?

The SQL INDEX hint suggests to the database query optimizer to use a specific index for a query, potentially improving query performance.

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Q.35 What is the SQL EXPLAIN statement used for?

The SQL EXPLAIN statement provides information about how the database query optimizer plans to execute a query, including the order of operations and access methods used.

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Q.36 What is the difference between a primary key and a unique key?

Both primary and unique keys enforce uniqueness, but a primary key also enforces the NOT NULL constraint, meaning it can't contain NULL values.

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Q.37 What is a SQL trigger?

A SQL trigger is a set of actions or code that is automatically executed in response to specific events or changes in a database, such as INSERT, UPDATE, or DELETE operations.

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Q.38 What is the purpose of the SQL LIMIT clause?

The SQL LIMIT clause is used to restrict the number of rows returned by a query. It is commonly used for pagination or to limit the result set for performance reasons.

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Q.39 What is the purpose of the SQL OFFSET clause?

The SQL OFFSET clause is used in combination with LIMIT to skip a specified number of rows before returning the remaining rows in the result set. It is also used for pagination.

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Q.40 What is a SQL full outer join?

A SQL full outer join retrieves all rows from both tables being joined. If there is no match in one table, NULL values are included for columns from the other table.

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Q.41 What is the SQL INTERSECT operator used for?

The SQL INTERSECT operator is used to retrieve rows that exist in both result sets of two or more SELECT queries. It returns the intersection of rows between queries.

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Q.42 What is the SQL UNION operator used for?

The SQL UNION operator combines the result sets of two or more SELECT queries into a single result set, removing duplicate rows from the combined result.

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Q.43 What is the SQL EXCEPT operator used for?

The SQL EXCEPT operator is used to retrieve rows from the first result set that do not exist in the second result set of two SELECT queries. It returns the set difference.

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Q.44 What is the SQL NULL value?

The SQL NULL value represents a missing or unknown data value in a database column. It is different from an empty string or zero and requires special handling in SQL queries.

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Q.45 What is the purpose of the SQL CASCADE option for foreign keys?

The CASCADE option, when used with foreign keys, automatically deletes or updates related rows in the referenced table when rows in the referencing table are deleted or updated.

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Q.46 What is a SQL self-reference foreign key?

A self-reference foreign key is a foreign key in a table that references the same table. It is used to represent hierarchical or recursive relationships within a single table.

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Q.47 What is the purpose of the SQL TRUNCATE TABLE statement?

The SQL TRUNCATE TABLE statement is used to remove all rows from a table quickly, but it does not log individual row deletions, making it faster than the DELETE statement.

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Q.48 What is the purpose of the SQL DROP TABLE statement?

The SQL DROP TABLE statement is used to delete an existing table and all its data, permanently removing the table structure and its associated data from the database.

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Q.49 What is the difference between the SQL INNER JOIN and LEFT JOIN?

An INNER JOIN retrieves matching rows from both tables, while a LEFT JOIN retrieves all rows from the left table and matching rows from the right table, filling non-matching rows with NULL values.

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Q.50 What is the SQL LIKE operator used for?

The SQL LIKE operator is used to filter rows based on a specified pattern in a column. It is often used with wildcard characters (%) to match partial strings.

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Q.51 What is the SQL NOT LIKE operator used for?

The SQL NOT LIKE operator is used to filter rows that do not match a specified pattern in a column. It is the opposite of the LIKE operator.

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Q.52 What is an SQL outer join?

An SQL outer join retrieves rows from one table and includes matching rows from another table, filling non-matching rows with NULL values for columns from the other table.

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Q.53 What is a SQL database transaction?

A SQL database transaction is a sequence of one or more SQL statements that are treated as a single unit of work. Transactions ensure data consistency and integrity.

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Q.54 What is the purpose of the SQL MERGE statement?

The SQL MERGE statement, also known as UPSERT, is used to perform INSERT, UPDATE, or DELETE operations on a target table based on a source table's data, using specified matching criteria.

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Q.55 What is the purpose of the SQL COALESCE function?

The SQL COALESCE function is used to return the first non-null value from a list of expressions. It is commonly used to handle NULL values and provide default values.

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Q.56 What is the SQL COUNT() function used for?

The SQL COUNT() function is used to count the number of rows in a result set or the number of non-null values in a specified column. It is often used with GROUP BY for aggregation.

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Q.57 What is the SQL MIN() function used for?

The SQL MIN() function is used to find the minimum value in a specified column of a result set. It is often used for finding the lowest value in a dataset.

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Q.58 What is the SQL MAX() function used for?

The SQL MAX() function is used to find the maximum value in a specified column of a result set. It is often used for finding the highest value in a dataset.

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Q.59 What is the SQL AVG() function used for?

The SQL AVG() function is used to calculate the average value of a specified column in a result set. It is often used for finding the mean value of a dataset.

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Q.60 What is the SQL SUM() function used for?

The SQL SUM() function is used to calculate the sum of values in a specified column of a result set. It is often used for finding the total of numeric values in a dataset.

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Q.61 What is the SQL BETWEEN operator used for?

The SQL BETWEEN operator is used to filter rows where a column's value falls within a specified range of values, including both the lower and upper bounds.

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Q.62 What is the SQL NOT BETWEEN operator used for?

The SQL NOT BETWEEN operator is used to filter rows where a column's value falls outside a specified range of values. It is the opposite of the BETWEEN operator.

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Q.63 What is the purpose of the SQL LIKE operator with ESCAPE?

The SQL LIKE operator with ESCAPE is used to search for patterns containing wildcard characters while escaping any character that should not be treated as a wildcard.

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Q.64 What is an SQL view and its advantages?

An SQL view is a virtual table created from the result of a SELECT query. Its advantages include data abstraction, security, and simplification of complex queries.

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Q.65 What are SQL transactions and their properties?

SQL transactions are sequences of SQL statements treated as a single unit. They have properties like ACID (Atomicity, Consistency, Isolation, Durability) for data integrity.

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Q.66 What is the purpose of the SQL COALESCE function?

The SQL COALESCE function is used to return the first non-null value from a list of expressions. It is commonly used to handle NULL values and provide default values.

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Q.67 What is the SQL COUNT() function used for?

The SQL COUNT() function is used to count the number of rows in a result set or the number of non-null values in a specified column. It is often used with GROUP BY for aggregation.

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Q.68 What is the SQL MIN() function used for?

The SQL MIN() function is used to find the minimum value in a specified column of a result set. It is often used for finding the lowest value in a dataset.

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Q.69 What is the SQL MAX() function used for?

The SQL MAX() function is used to find the maximum value in a specified column of a result set. It is often used for finding the highest value in a dataset.

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Q.70 What is the SQL AVG() function used for?

The SQL AVG() function is used to calculate the average value of a specified column in a result set. It is often used for finding the mean value of a dataset.

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Q.71 What is the SQL SUM() function used for?

The SQL SUM() function is used to calculate the sum of values in a specified column of a result set. It is often used for finding the total of numeric values in a dataset.

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Q.72 What is the SQL BETWEEN operator used for?

The SQL BETWEEN operator is used to filter rows where a column's value falls within a specified range of values, including both the lower and upper bounds.

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Q.73 What is the SQL NOT BETWEEN operator used for?

The SQL NOT BETWEEN operator is used to filter rows where a column's value falls outside a specified range of values. It is the opposite of the BETWEEN operator.

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Q.74 What is the purpose of the SQL LIKE operator with ESCAPE?

The SQL LIKE operator with ESCAPE is used to search for patterns containing wildcard characters while escaping any character that should not be treated as a wildcard.

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Q.75 What is an SQL view and its advantages?

An SQL view is a virtual table created from the result of a SELECT query. Its advantages include data abstraction, security, and simplification of complex queries.

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Q.76 What are SQL transactions and their properties?

SQL transactions are sequences of SQL statements treated as a single unit. They have properties like ACID (Atomicity, Consistency, Isolation, Durability) for data integrity.

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Q.77 What is the purpose of the SQL CASE statement?

The SQL CASE statement is used for conditional logic in SQL queries. It allows you to perform different actions based on specified conditions within a single query.

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Q.78 What is the SQL UNION ALL operator used for?

The SQL UNION ALL operator combines the result sets of two or more SELECT queries into a single result set, including all rows, even if they are duplicates.

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Q.79 What is the SQL INTERSECT operator used for?

The SQL INTERSECT operator is used to retrieve rows that exist in both result sets of two or more SELECT queries. It returns the intersection of rows between queries.

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Q.80 What is the purpose of the SQL MERGE statement?

The SQL MERGE statement, also known as UPSERT, is used to perform INSERT, UPDATE, or DELETE operations on a target table based on a source table's data, using specified matching criteria.

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Q.81 What is the SQL CONCATENATION operator used for?

The SQL CONCATENATION operator, often represented as the plus sign (+) or double pipe (||).

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Q.82 What is the purpose of the SQL IN operator?

The SQL IN operator is used to filter rows where a column's value matches any value in a specified list or subquery. It is a shorthand for multiple OR conditions.

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Q.83 What is the SQL EXISTS operator used for?

The SQL EXISTS operator is used to check if a subquery returns any rows. It is often used in correlated subqueries to conditionally filter rows based on subquery results.

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Q.84 What is the SQL UNION ALL operator used for?

The SQL UNION ALL operator combines the result sets of two or more SELECT queries into a single result set, including all rows, even if they are duplicates.

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Q.85 What is the SQL INTERSECT operator used for?

The SQL INTERSECT operator is used to retrieve rows that exist in both result sets of two or more SELECT queries. It returns the intersection of rows between queries.

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Q.86 What is the purpose of the SQL MERGE statement?

The SQL MERGE statement, also known as UPSERT, is used to perform INSERT, UPDATE, or DELETE operations on a target table based on a source table's data, using specified matching criteria.

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Q.87 What is the purpose of the SQL IN operator?

The SQL IN operator is used to filter rows where a column's value matches any value in a specified list or subquery. It is a shorthand for multiple OR conditions.

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Q.88 What is the SQL EXISTS operator used for?

The SQL EXISTS operator is used to check if a subquery returns any rows. It is often used in correlated subqueries to conditionally filter rows based on subquery results.

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Q.89 What is the SQL LIMIT clause used for?

The SQL LIMIT clause is used to restrict the number of rows returned by a query, allowing for pagination or limiting the result set for performance reasons.

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Q.90 What is the SQL OFFSET clause used for?

The SQL OFFSET clause is used in combination with LIMIT to skip a specified number of rows before returning the remaining rows in the result set. It is used for pagination.

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Q.91 What is the SQL COALESCE function used for?

The SQL COALESCE function is used to return the first non-null value from a list of expressions. It is commonly used to handle NULL values and provide default values.

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Q.92 What is the SQL COUNT() function used for?

The SQL COUNT() function is used to count the number of rows in a result set or the number of non-null values in a specified column. It is often used with GROUP BY for aggregation.

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Q.93 What is the SQL MIN() function used for?

The SQL MIN() function is used to find the minimum value in a specified column of a result set. It is often used for finding the lowest value in a dataset.

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Q.94 What is the SQL MAX() function used for?

The SQL MAX() function is used to find the maximum value in a specified column of a result set. It is often used for finding the highest value in a dataset.

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Q.95 What is the SQL AVG() function used for?

The SQL AVG() function is used to calculate the average value of a specified column in a result set. It is often used for finding the mean value of a dataset.

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Q.96 What is the SQL SUM() function used for?

The SQL SUM() function is used to calculate the sum of values in a specified column of a result set. It is often used for finding the total of numeric values in a dataset.

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Q.97 What is the SQL BETWEEN operator used for?

The SQL BETWEEN operator is used to filter rows where a column's value falls within a specified range of values, including both the lower and upper bounds.

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Q.98 What is the SQL NOT BETWEEN operator used for?

The SQL NOT BETWEEN operator is used to filter rows where a column's value falls outside a specified range of values. It is the opposite of the BETWEEN operator.

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Q.99 What is the purpose of the SQL LIKE operator with ESCAPE?

The SQL LIKE operator with ESCAPE is used to search for patterns containing wildcard characters while escaping any character that should not be treated as a wildcard.

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Q.100 What is an SQL view and its advantages?

An SQL view is a virtual table created from the result of a SELECT query. Its advantages include data abstraction, security, and simplification of complex queries.

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Q.101 What are SQL transactions and their properties?

SQL transactions are sequences of SQL statements treated as a single unit. They have properties like ACID (Atomicity, Consistency, Isolation, Durability) for data integrity.

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Q.102 What is the purpose of the SQL CASE statement?

The SQL CASE statement is used for conditional logic in SQL queries. It allows you to perform different actions based on specified conditions within a single query.

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Q.103 What is the SQL UNION ALL operator used for?

The SQL UNION ALL operator combines the result sets of two or more SELECT queries into a single result set, including all rows, even if they are duplicates.

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