What does case mean in sql

The CASE statement is a SQL control structure that executes different SQL statements based on conditional expressions and returns results. The advantages include: providing conditional dynamic results, decomposing complex conditions, simplicity and efficiency.

The meaning of the CASE statement in SQL

The CASE statement is a SQL control structure that allows Conditions perform different actions. It executes a set of SQL statements based on one or more conditional expressions and returns a result.

Structure

The syntax of the CASE statement is as follows:

<code>CASE
    WHEN 条件表达式1 THEN 结果表达式1
    WHEN 条件表达式2 THEN 结果表达式2
    ...
    ELSE 默认结果表达式
END</code>

Usage

1. Conditional expression: It is a Boolean expression used to determine whether to execute the corresponding SQL statement.
2. Result expression: If the conditional expression is true, this SQL statement is executed and the result is returned.
3. ELSE clause: If all conditional expressions are false, this SQL statement is executed and the result is returned. It is optional.

Example

<code class="sql">SELECT CASE
    WHEN age > 18 THEN '成年'
    WHEN age < 18 THEN '未成年'
    ELSE '非法年龄'
END AS age_category
FROM persons;</code>

Executing this query will set the age_category column to "adult", "minor", or "illegal age" based on each person's age .

Advantages

The main advantages of the CASE statement include:

• Provides the flexibility to dynamically generate results based on conditions.
• Complex conditions can be easily broken down into simpler conditional expressions.
• More concise and efficient than using IF-ELSE statement.

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