sql典例分析

1. 条件过滤 & Having

表结构

#tab_a

#tab_b

表关系

tab_a.id = tab_b.relation_id

表数据

需求

查新把tab_a的ID对应的表tab_b的member_id找出来，当在表tab_b中找不到时，赋值为null

select tab_a.id,
 case
     when (SELECT count(member_id) from tab_b WHERE relation_id = tab_a.id ) = 0 then null
     ELSE (SELECT member_id from tab_b WHERE relation_id = tab_a.id )
 end as result
from tab_a

结果

问题

当tab_a的id与tab_b的member_id存在1对多的关系时就不行了，如tab_b

再做上面的sql查询是出现错误：

[Err] ERROR:  more than one row returned by a subquery used as an expression

问题

如何找到tab_a中的id 对应的tab_b中的member_id有多个？

select tab_a.id, count(*) from tab_a, tab_b where tab_a.id=tab_b.relation_id group by tab_a.id having count(*)>1

结果

2. 一个表的结构作为另一个表的条件

update orders p, (select order_id, sum(count) as cnt from job_info group by order_id) b set p.count_raw = b.cnt where p.id =b.order_id;