分組
- 按照字段分組,表示此字段相同的數據回被放到一個組中
- 分組後,只能查詢出相同的數據列
- 可以對分組後的數據進行統計,做聚合運算
- 語法:
select 列1,列2,聚合... from 表名 group by 列1,列2,列3...
- 查詢男女生總數
select gender,count(*) from students group by gender;
分組後進行篩選
- 查詢
select gender,count(*) from students group by gender;
- 查詢男女生總數
select gender,count(*) from students group by gender;
- 分組後篩選 gender=0
select gender,count(*) from students group by gender having gender=0;
起別名:
select gender,count(*) as rs from students group by gender having rs>2;
where和having區別
都是篩選,面向的數據集不一樣的。
- where是對原始數據集進行篩選
- having是對group by之後形成的結果集進行篩選