- 问题描述:left join副表查询条件影响了主表查询结果
sql代码如下:
SELECT count(r.id) as weight ,r.*,m.*
FROM room r left join meeting m on m.room_id =r.id
where r.tenant_id =1 and r.is_del = 0 and (m.organizer_id = 7 OR m.organizer_id IS NULL ) GROUP BY r.id,r.name
如上,meeting表是left join进来的表,room表是主表,按理说meeting表的判断条件不会影响主表,但是实际测试发现:
sql语句1:
SELECT count(r.id) as weight ,r.*,m.*
FROM room r left join meeting m on m.room_id =r.id
where r.tenant_id =1 and r.is_del = 0 GROUP BY r.id,r.name
查出来的数据有8条;
sql语句2:
SELECT count(r.id) as weight ,r.*,m.*
FROM room r left join meeting m on m.room_id =r.id
where r.tenant_id =1 and r.is_del = 0 and (m.organizer_id = 7 OR m.organizer_id IS NULL ) GROUP BY r.id,r.name
查出来的数据有7条;
即:
m.organizer_id = 7 OR m.organizer_id IS NULL
这个查询条件过滤掉了一条room表的数据。
这里看到这个查询条件加在where后面,现在把他提前,放到on后where前,即:
SELECT count(r.id) as weight ,r.*,m.*
FROM room r left join meeting m on m.room_id =r.id and (m.organizer_id = 7 OR m.organizer_id IS NULL )
where r.tenant_id =1 and r.is_del = 0 GROUP BY r.id,r.name
查出来的就是8条了
- 结论:left join联表查询,放在on后的查询条件才不会影响主表的查询结果