MySQL的7種連接徹底清楚了！

不說話先看圖吧：
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SQL的7種連接分別是：inner join、left join、right join、full outer join（2種），但MySQL的full outer join方式卻有些不一樣。

1. 內連接（交集）

   select * from x_user a inner join x_school b on a.id=b.id;
   

2. 左外連接

   select * from x_user a left join x_school b on a.id=b.id;
   

3. 右外連接

   	select * from x_user a right join x_school b on a.id=b.id;
   

4. 左差集

   select * from x_user a left join x_school b on a.id=b.id where b.id is null;
   

5. 右差集

   select * from x_user a right join x_school b on a.id=b.id where b.id is null;
   

6. 並集

   select * from x_user a left join x_school b on a.id=b.id union select * from x_user a right join x_school b on a.id=b.id;
   

   並集就是交叉和不交叉的都顯示出來，但需要注意的是union左右兩邊查詢的字段個數要保持一致；另外如果同一列下，union左側顯示左邊的字段，右邊顯示的寫右邊的字段，那麼需要在左右分別顯示的寫上各自的字段就好了，比如下面的1.drugstore_name, t1.organ_sign, t1.province, t1.city和 t2.drugstore_name, t2.organ_sign, t2.province, t2.city是一一對應的，那麼查詢出來的就是各自的字段，如下表格圖（紅色畫框的是表t2字段的數據）。

   SELECT
   t1.drugstore_name,
   t1.organ_sign,
   t1.province,
   t1.city,
   t1.order_id sku_10_order_id,
   t1.member_guid sku_10_member_guid,
   t1.sku_10_amount,
   t2.order_id sku_1_order_id,
   t2.member_guid sku_1_member_guid,
   t2.sku_1_amount 
   FROM	t1	LEFT JOIN t2 ON t1.organ_sign = t2.organ_sign 	AND t1.order_id = t2.order_id 
   UNION
   SELECT
   t2.drugstore_name,
   t2.organ_sign,
   t2.province,
   t2.city,
   t1.order_id,
   t1.member_guid,
   t1.sku_10_amount,
   t2.order_id sku_1_order_id,
   t2.member_guid sku_1_member_guid,
   t2.sku_1_amount 
   FROM
   t1	RIGHT JOIN t2 ON t1.organ_sign = t2.organ_sign 	AND t1.order_id = t2.order_id;
   

7. 差集

   select * from x_user a left join x_school b on a.id=b.id where b.id is  null union select * from x_user a right join x_school b on a.id=b.id where a.id is null;