MySQL数据库之多表查询cross join交叉连接

交叉连接

• 语法
  • select * from 表1 cross join 表2 on ...
  • 交叉连接如果没有连接条件返回笛卡尔积
  • 如果有连接条件和内连接是一样的

MariaDB [sel]> select * from grades cross join resume;
+-------+---------+------+----+-------+-----------+
| name  | chinese | math | id | name  | skill     |
+-------+---------+------+----+-------+-----------+
| Sunny |      93 |   96 |  1 | Sunny | php       |
| Sunny |      93 |   96 |  2 | Kimmy | php       |
| Sunny |      93 |   96 |  3 | Jerry | php,mysql |
| Jerry |      97 |   91 |  1 | Sunny | php       |
| Jerry |      97 |   91 |  2 | Kimmy | php       |
| Jerry |      97 |   91 |  3 | Jerry | php,mysql |
| Marry |      95 |   94 |  1 | Sunny | php       |
| Marry |      95 |   94 |  2 | Kimmy | php       |
| Marry |      95 |   94 |  3 | Jerry | php,mysql |
| Tommy |      98 |   94 |  1 | Sunny | php       |
| Tommy |      98 |   94 |  2 | Kimmy | php       |
| Tommy |      98 |   94 |  3 | Jerry | php,mysql |
+-------+---------+------+----+-------+-----------+
# `12 rows in set (0.001 sec)`

• 交叉连接有连接表达式与内连接是一样的

MariaDB [sel]> select * from grades cross join resume on grades.name=resume.name;
+-------+---------+------+----+-------+-----------+
| name  | chinese | math | id | name  | skill     |
+-------+---------+------+----+-------+-----------+
| Sunny |      93 |   96 |  1 | Sunny | php       |
| Jerry |      97 |   91 |  3 | Jerry | php,mysql |
+-------+---------+------+----+-------+-----------+
# `2 rows in set (0.000 sec)`