mysql order by case when

mysql order by case when的用法

需求

对一个年级的学生按班级进行排序，按班级从小到大排序，如果班级为空排在最后

代码

SELECT
    @ROW := @ROW + 1 AS rowno,
    ID,
    MOBILE_NO,
    FIRST_NAME,
    MEM_CLASS,
    enrollment_year,
    DATE_FORMAT(joinclass_date, '%Y-%m-%d') joinclass_date,
    NIKE_NAME,
    LOGIN_NAME
FROM
    T_MEMBER,
    (SELECT @ROW := 0) r
WHERE
    DELETED = 'N'
AND MEM_TYPE = 1
AND (
    MEM_CLASS IS NULL
    OR MEM_CLASS = ''
    OR MEM_CLASS NOT IN (1, 2, 3, 4, 5, 6)
)
AND SCHOOLID = 11839
AND enrollment_year = 2016
ORDER BY
    CASE
WHEN ifnull(MEM_CLASS, '') = '' THEN
    0
ELSE
    1
END DESC,
 - MEM_CLASS DESC

解读

由于MEM_CLASS 字段被设置成了字符串类型，但我需要按数字从小到大排序，所以MEM_CLASS 前面加个负号“-”，将字符转成数字

下面是重点

其实是个组合排序
ORDER BY
    -- 先按CASE WHEN的结果（ifnull(MEM_CLASS, '') = '' THEN   0 ELSE  1 END）即0和1进行降序排列
    CASE WHEN   ifnull(MEM_CLASS, '') = '' THEN 0 ELSE  1 END DESC,
    -- 再按‘- MEM_CLASS’进行降序排列（即数字升序排列）
     - MEM_CLASS DESC