Usage of casewhen in sql

The CASE WHEN statement is used to return different values ​​based on conditions. Its usage steps are: Specify conditions: Determine the conditions to be evaluated. Specifies the result: the value returned if the condition is true. Specify an ELSE clause: If no condition is true, a default value is returned (optional).

Usage of CASE WHEN statement in SQL

The CASE WHEN statement is a conditional statement in SQL query language An expression that returns a different value based on specified conditions. The syntax is as follows:

<code class="sql">CASE
  WHEN condition1 THEN result1
  WHEN condition2 THEN result2
  ...
  ELSE default_result
END</code>

Usage steps:

1. Specify conditions (condition): Determine the conditions to be evaluated.
2. Specify the result (result): The value returned if the condition is true.
3. Specify the ELSE clause: If no condition is true, return the default value (optional).

Example:

<code class="sql">SELECT CASE
  WHEN age > 18 THEN '成年'
  WHEN age >= 13 AND age <= 18 THEN '青少年'
  ELSE '儿童'
END AS age_category
FROM students;</code>

Result:

##1John20Adult2Mary16teenager3Peter10Children

Student ID	Name	Age	Age Category

Other uses:

• Return NULL value: The ELSE clause can be set to NULL to return a null value when no condition is true.
• Nested CASE WHEN statements: CASE WHEN statements can be nested to handle more complex conditions.
• Use CASE expressions as filter conditions: You can use CASE expressions in the WHERE clause to filter data.

Advantages:

Simplifies queries with complex conditions.
• Improves the performance of queries because the server does not need to execute multiple IF-THEN statements.
• Provides a concise way to handle different situations in SQL queries.

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