简化If和else IF查询

CASE expression
    WHEN value1 THEN result1
    WHEN value2 THEN result2
    ...
    ELSE result
END

SELECT
    CASE gender
        WHEN '1' THEN '男性'
        WHEN '2' THEN '女性'
        WHEN '3' THEN '其他'
        ELSE '未知'
    END AS '性别'
FROM
    表名;

CASE
    WHEN condition1 THEN result1
    WHEN condition2 THEN result2
    ...
    ELSE result
END

SELECT
    CASE
        WHEN score >= 90 THEN '优秀'
        WHEN score >= 80 THEN '良好'
        WHEN score >= 70 THEN '中等'
        WHEN score >= 60 THEN '及格'
        ELSE '不及格'
    END AS '等级'
FROM
    表名;