本文详细介绍了 SQL 中 Group By 子句的使用方法，包括其语法结构、执行顺序及编写过程。并强调了当 SQL 语句中包含 Group By 时，Select 子句中只能出现被 Group By 列或聚合函数。
sql带有group by，那么select里头只能包含group by by的列或者是聚合函数
select max(id) id, catid from post group by catid order by id desc;
写的顺序：select ... from... where.... group by... having... order by..
执行顺序：from... where...group by... having.... select ... order by...
编写过程：select distinct ... from ... join ... on ... where.... group by ... having ... order by ... limit ...
解析过程：from ... on ... join ... where.... group by... having ... select distinct ... order by... limit ...
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