数据库mysql存储中case如何转化条件?

CASE 表达式
    WHEN 值1 THEN 结果1
    WHEN 值2 THEN 结果2
    ...
    ELSE 默认结果
END

SELECT 
    user_id,
    level,
    CASE level
        WHEN 1 THEN '初级'
        WHEN 2 THEN '中级'
        WHEN 3 THEN '高级'
        ELSE '未知等级'
    END AS level_desc
FROM users;

CASE
    WHEN 条件1 THEN 结果1
    WHEN 条件2 THEN 结果2
    ...
    ELSE 默认结果
END

SELECT 
    student_id,
    score,
    CASE
        WHEN score >= 90 THEN '优秀'
        WHEN score >= 80 THEN '良好'
        WHEN score >= 60 THEN '及格'
        ELSE '不及格'
    END AS grade
FROM scores;

-- ❌ 错误：不能这样写
WHERE CASE role 
         WHEN 'admin' THEN id > 0 
         ELSE id > 100 
     END

WHERE (role = 'admin' AND id > 0)
   OR (role != 'admin' AND id > 100)

SELECT *
FROM (
    SELECT *,
        CASE 
            WHEN role = 'admin' THEN 'A'
            ELSE 'B'
        END AS category
    FROM users
) t
WHERE category = 'A';

SELECT name, status
FROM tasks
ORDER BY 
    CASE status
        WHEN 'urgent' THEN 1
        WHEN 'pending' THEN 2
        WHEN 'done' THEN 3
        ELSE 4
    END;

ORDER BY 
    CASE 
        WHEN active = 1 THEN sort_order 
        ELSE 999 
    END;