mysql 查询语句case

-- 简单 CASE 示例
SELECT 
    id,
    CASE status
        WHEN 'active' THEN '启用'
        WHEN 'inactive' THEN '禁用'
        ELSE '未知'
    END AS status_text
FROM users;

-- 搜索 CASE 示例
SELECT 
    id,
    CASE
        WHEN age < 18 THEN '未成年'
        WHEN age >= 18 AND age < 60 THEN '成年'
        ELSE '老年'
    END AS age_group
FROM users;

-- 条件聚合示例
SELECT 
    department,
    SUM(CASE WHEN salary > 5000 THEN 1 ELSE 0 END) AS high_salary_count,
    SUM(CASE WHEN salary <= 5000 THEN 1 ELSE 0 END) AS low_salary_count
FROM employees
GROUP BY department;

-- 行转列示例
SELECT 
    id,
    MAX(CASE WHEN category = 'A' THEN value END) AS A_value,
    MAX(CASE WHEN category = 'B' THEN value END) AS B_value
FROM data
GROUP BY id;