a b ca.pid =b.pidb.prov=c.pid
groupby a.moon
比如SELECT t2.group_id,count(*) as gnm from A t1 LEFT JOIN B t2 on t1.id=t2.id LEFT JOIN C t3 on t2.id=t3.idGROUP BY t2.group_id。执行逻辑就是先查询,再GROUP BY主要你要搞清楚group by 后多个参数的含义。
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