mysql join

数据库中join又称连接，用于联合多表的查询。

我们会以两张表做例子customer：

mysql> select * from customer;
+------------+-----------+
| customerid | name      |
+------------+-----------+
|          1 | Joe       |
|          2 | Christian |
|          3 | Zero      |
|          4 | Karl      |
+------------+-----------+
4 rows in set (0.00 sec)

mysql> select * from `order`;
+---------+------------+
| orderid | customerid |
+---------+------------+
|   10001 |          1 |
|   10002 |          2 |
|   10003 |          3 |
|   10005 |          5 |
+---------+------------+
4 rows in set (0.00 sec)

inner join内连接

inner join或者两个表中字段匹配关系的记录。

select * from customer INNER JOIN `order` on customer.customerid=`order`.customerid;
+------------+-----------+---------+------------+
| customerid | name      | orderid | customerid |
+------------+-----------+---------+------------+
|          1 | Joe       |   10001 |          1 |
|          2 | Christian |   10002 |          2 |
|          3 | Zero      |   10003 |          3 |
+------------+-----------+---------+------------+
3 rows in set (0.00 sec)

left join左连接

获取左边所有记录，即使右表没有对应的匹配的记录。

mysql> select * from customer left JOIN `order` on customer.customerid=`order`.customerid;
+------------+-----------+---------+------------+
| customerid | name      | orderid | customerid |
+------------+-----------+---------+------------+
|          1 | Joe       |   10001 |          1 |
|          2 | Christian |   10002 |          2 |
|          3 | Zero      |   10003 |          3 |
|          4 | Karl      |    NULL |       NULL |
+------------+-----------+---------+------------+
4 rows in set (0.00 sec)

right join右连接

与left join相反，用于获取右边所有记录，即使左边没有对应匹配的记录。

mysql> select * from customer right JOIN `order` on customer.customerid=`order`.customerid;
+------------+-----------+---------+------------+
| customerid | name      | orderid | customerid |
+------------+-----------+---------+------------+
|          1 | Joe       |   10001 |          1 |
|          2 | Christian |   10002 |          2 |
|          3 | Zero      |   10003 |          3 |
|       NULL | NULL      |   10005 |          5 |
+------------+-----------+---------+------------+
4 rows in set (0.00 sec)

LEFT JOIN EXCLUDING INNER JOIN (左连接 - 内连接)

mysql> select * from customer LEFT JOIN `order` on customer.customerid=`order`.customerid where `order`.customerid IS NULL;
+------------+------+---------+------------+
| customerid | name | orderid | customerid |
+------------+------+---------+------------+
|          4 | Karl |    NULL |       NULL |
+------------+------+---------+------------+
1 row in set (0.00 sec)

RIGHT JOIN EXCLUDING INNER JOIN (右连接 - 内连接)

mysql> select * from customer RIGHT JOIN `order` on customer.customerid=`order`.customerid where `customer`.customerid IS NULL;
+------------+------+---------+------------+
| customerid | name | orderid | customerid |
+------------+------+---------+------------+
|       NULL | NULL |   10005 |          5 |
+------------+------+---------+------------+
1 row in set (0.00 sec)

CROSS JOIN笛卡尔积

左边的每一条记录和右边每一条记录组成数据对。

mysql> select * from customer CROSS JOIN `order`;
+------------+-----------+---------+------------+
| customerid | name      | orderid | customerid |
+------------+-----------+---------+------------+
|          1 | Joe       |   10001 |          1 |
|          2 | Christian |   10001 |          1 |
|          3 | Zero      |   10001 |          1 |
|          4 | Karl      |   10001 |          1 |
|          1 | Joe       |   10002 |          2 |
|          2 | Christian |   10002 |          2 |
|          3 | Zero      |   10002 |          2 |
|          4 | Karl      |   10002 |          2 |
|          1 | Joe       |   10003 |          3 |
|          2 | Christian |   10003 |          3 |
|          3 | Zero      |   10003 |          3 |
|          4 | Karl      |   10003 |          3 |
|          1 | Joe       |   10005 |          5 |
|          2 | Christian |   10005 |          5 |
|          3 | Zero      |   10005 |          5 |
|          4 | Karl      |   10005 |          5 |
|          1 | Joe       |   10004 |          1 |
|          2 | Christian |   10004 |          1 |
|          3 | Zero      |   10004 |          1 |
|          4 | Karl      |   10004 |          1 |
+------------+-----------+---------+------------+
20 rows in set (0.00 sec)

参考：https://blog.csdn.net/zhengsy_/article/details/90733864

更多讲解,欢迎关注我的github:
go成神之路