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                合規國際互聯網加速 OSASE為企業客戶提供高速穩定SD-WAN國際加速解決方案。 廣告
                ## 分組 * 按照字段分組,表示此字段相同的數據會被放到一個組中 * 分組后,只能查詢出相同的數據列,對于有差異的數據列無法出現在結果集中 * 可以對分組后的數據進行統計,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 as 性別,count(*) from students group by gender; ~~~ * 查詢各城市人數 ~~~ select hometown as 家鄉,count(*) from students group by hometown; ~~~ ## 分組后的數據篩選 * 語法: ~~~ select 列1,列2,聚合... from 表名 group by 列1,列2,列3... having 列1,...聚合... ~~~ * having后面的條件運算符與where的相同 * 查詢男生總人數 ~~~ 方案一 select count(*) from students where gender=1; ----------------------------------- 方案二: select gender as 性別,count(*) from students group by gender having gender=1; ~~~ ## 對比where與having * where是對from后面指定的表進行數據篩選,屬于對原始數據的篩選 * having是對group by的結果進行篩選
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