表连接查询本质是以左边的那个表为准
举例：
mysql> select * from t1;
+------+-------+
| name | value |
+------+-------+
| a | 5 |
| b | 10 |
| c | 15 |
| d | 10 |
+------+-------+
4 rows in set (0.00 sec)
mysql> select * from t2;
+------+-------+
| name | value |
+------+-------+
| b | 5 |
| c | 10 |
| d | 20 |
| e | 99 |
| a | 8 |
| a | 7 |
+------+-------+
6 rows in set (0.00 sec)
mysql> select * from t1
-> left join t2
-> on t1.name = t2.name;
+------+-------+------+-------+
| name | value | name | value |
+------+-------+------+-------+
| b | 10 | b | 5 |
| c | 15 | c | 10 |
| d | 10 | d | 20 |
| a | 5 | a | 8 |
| a | 5 | a | 7 |
+------+-------+------+-------+
5 rows in set (0.00 sec)
此时因为是left join，从结果也能明显的看出是以左边的为基准
mysql> select * from t1
-> right join t2
-> on t1.name = t2.name;
+------+-------+------+-------+
| name | value | name | value |
+------+-------+------+-------+
| a | 5 | a | 8 |
| a | 5 | a | 7 |
| b | 10 | b | 5 |
| c | 15 | c | 10 |
| d | 10 | d | 20 |
| NULL | NULL | e | 99 |
+------+-------+------+-------+
6 rows in set (0.00 sec)
此时right join，所以是以t2为基准的，
t1右连接t2，所以此时t2是在左边的
归根结底：连接查询是以左边的那个表为基准的！
来自 “ ITPUB博客 ” ，链接：https://blog.itpub.net/28212711/viewspace-758979/ ，如需转载，请注明出处，否则将追究法律责任。
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