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SQL 中 CASE WHEN 及 SELECT CASE WHEN 的用法

news 2025/7/25 20:43:06

SQL 中 CASE WHEN 及 SELECT CASE WHEN 的用法

CASE WHEN 是 SQL 中非常实用的条件表达式，它允许你在查询中实现条件逻辑。以下是详细的用法说明：

1. 基本语法结构

CASE WHEN condition1 THEN result1WHEN condition2 THEN result2...ELSE default_result
END

或者简单形式：

CASE expression WHEN value1 THEN result1WHEN value2 THEN result2...ELSE default_result
END

2. 在 SELECT 语句中的用法

示例1：简单条件判断

SELECT employee_name,salary,CASE WHEN salary > 10000 THEN '高薪'WHEN salary > 5000 THEN '中等'ELSE '一般'END AS salary_level
FROM employees;

示例2：等值比较（简单形式）

SELECT product_name,CASE category_idWHEN 1 THEN '电子产品'WHEN 2 THEN '服装'WHEN 3 THEN '食品'ELSE '其他'END AS category_name
FROM products;

3. 在 UPDATE 语句中的用法

UPDATE orders
SET discount = CASE WHEN order_amount > 1000 THEN 0.1WHEN order_amount > 500 THEN 0.05ELSE 0END;

4. 在 ORDER BY 中的用法

SELECT * FROM customers
ORDER BY CASE WHEN country = 'China' THEN 1WHEN country = 'USA' THEN 2ELSE 3END;

5. 在 GROUP BY 中的用法

SELECT CASE WHEN age < 20 THEN '青少年'WHEN age BETWEEN 20 AND 30 THEN '青年'ELSE '成年'END AS age_group,COUNT(*) AS count
FROM users
GROUP BY age_group;

6. 嵌套 CASE WHEN 用法

SELECT student_name,score,CASE WHEN score >= 90 THEN 'A'WHEN score >= 80 THEN CASE WHEN attendance_rate > 0.9 THEN 'B+'ELSE 'B'ENDWHEN score >= 70 THEN 'C'ELSE 'D'END AS grade
FROM students;

7. 在聚合函数中使用

SELECT department_id,SUM(CASE WHEN gender = 'M' THEN 1 ELSE 0 END) AS male_count,SUM(CASE WHEN gender = 'F' THEN 1 ELSE 0 END) AS female_count
FROM employees
GROUP BY department_id;