Mysql函数

一，列表分组之后查询所有组内某一个时间字段最大的数据

ContractEndDate ：某一个时间字段
RECORDID：表ID

SELECT A.* from (select * from (SELECT * FROM hrm_contract order by ContractEndDate DESC limit 10000) a GROUP BY a.A00) B
 left join hrm_contract A on a.RECORDID=b.RECORDID

二，多级菜单表根据parent_id查询包括父节点在内的所有子节点id

SELECT * FROM (

SELECT t1.id,IF(FIND_IN_SET(t1.parent_id,@pids)>0,@pids:=CONCAT(@pids,',',id),0) as ischild FROM (
SELECT * FROM menu  ORDER BY  parent_id
) t1,(SELECT @pids:=1) t2

UNION
SELECT dd.id ,dd.id as ischild FROM menu as dd WHERE dd.id=1
) cc WHERE cc.ischild <> 0

三，多级菜单表根据子节点id查询包括子节点在内的所有父节点parent_id

表结构

查询sql

SELECT T2.id
FROM ( 
    SELECT 
        @r AS _id, 
        (SELECT @r := parent_id FROM user WHERE id = _id) AS parent_id, 
        @l := @l + 1 AS lvl 
    FROM 
        (SELECT @r := 7, @l := 0) vars,  -- 查询id为5的所有上级
        user h 
    WHERE @r <> 0) T1 
JOIN user T2 
ON T1._id = T2.id 
ORDER BY T1.lvl DESC

查询结果

四，group by 对多个字段进行分组

group by 字段一：将所有具有相同字段一的记录放到一个分组里

group by 字段一，字段二 ：将所有具有相同字段一和字段二的记录放到一个分组里

五，Case When和聚合函数count、sum的使用

根据贫困标志，取出不同标准下的的人数有多少，以区县位单位。

1代表已脱贫人口；2代表返贫人口；3代表未脱贫

select 
t3.region_id as 'regionId',
t3.region_name as 'regionName',
count(case when t1.tricolor = 1 then 1 else null end ) as 'greenTotal',
count(case when t1.tricolor = 2 then 1 else null end ) as 'yellowTotal',
count(case when t1.tricolor = 3 then 1 else null end )as 'redTotal'
from pa_household_member t1
left join pa_household t2 on t2.household_id = t1.household_id
left join system_region t3 on t3.region_id = t2.area 
group by t2.area

count(1) 算上一条记录 count（null）为0；利用此特性进行分类统计。不用写复杂的子查询。count在套住case when ..... 例如： count (case when ....end )

原文地址：https://www.cnblogs.com/fantongxue/p/13680068.html