1，group by 創建分組

在mysql中 group by 句子要註意，必須where 之後，order by 之前

select order_code ,count(*) as product from order group by order_code;

2,having 分組過濾

where子句都可以用having代替，區別在於where過濾行，having過濾分組；having支持所有的where操作符

但where不能用於替換having 分組濾

select order_code ,count(*) as product from order group by order_code having count(*)>2;

having和where的區別：

where在數據分組前進行過濾，having在數據分組後進行過濾；where排除的行不包括在分組中（這可能會改變計算值，從而影響having子句中基於這些值過濾掉的分組）

mysql group by 分組及having 過濾分組