Usage of if statement after where in sql

The IF statement is used in the SQL WHERE clause to create a conditional expression to perform different operations based on a certain condition. It can replace null values ​​with another value, return different values ​​based on conditions, and perform nested queries based on different conditions.

Usage of IF statement in WHERE clause in SQL

WHERE clause is used to filter in SQL query Data, returning only rows that meet the specified criteria. The IF statement can be used to create conditional expressions in the WHERE clause to perform different actions based on a condition.

IF statement syntax

<code>WHERE IF(condition, true_value, false_value)</code>

Where:

• condition: The Boolean expression to be evaluated.
• true_value: The value returned if the condition is true.
• false_value: The value returned if the condition is false.

Usage

The IF statement can be used to create complex filter conditions, which can perform the following operations in the WHERE clause:

• Replace a null value with another value.
• Return different values ​​based on conditions.
• Execute nested queries based on different conditions.

Example

Replace null values ​​with default values:

<code>SELECT *
FROM table_name
WHERE IF(column_name IS NULL, 'N/A', column_name);</code>

This will return a table where All column_name columns with null values ​​will be replaced with "N/A".

Return different values ​​according to the conditions:

<code>SELECT *
FROM table_name
WHERE IF(age >= 18, 'Adult', 'Underage');</code>

This will return a table in which the value of the age column is greater than or equal to 18 as "Adult", and the behavior as less than 18 "Underage".

Execute a nested query based on different conditions:

<code>SELECT *
FROM table_name
WHERE IF(country = 'USA', (SELECT MAX(salary) FROM employees WHERE country = 'USA'),
                              (SELECT MAX(salary) FROM employees WHERE country = 'UK'));</code>

This will return a table where the value of the salary column is the maximum salary of a US employee, if it is a UK employee , is the maximum salary for UK employees.

The above is the detailed content of Usage of if statement after where in sql. For more information, please follow other related articles on the PHP Chinese website!