我们只能根据连接谓词（即指定的条件）来区分 MySQL CROSS JOIN 和 INNER JOIN。在编写 INNER JOIN 查询时，我们需要指定条件，但相反，在编写 CROSS JOIN 查询时，我们不需要指定条件。为了理解它，我们以名为 tbl_1 和 tbl_2 的两个表为例，它们具有以下数据 -
mysql> Select * from tbl_1;
+----+--------+
| Id | Name |
+----+--------+
| 1 | Gaurav |
| 2 | Rahul |
| 3 | Raman |
| 4 | Aarav |
+----+--------+
4 rows in set (0.00 sec)
mysql> Select * from tbl_2;
+----+---------+
| Id | Name |
+----+---------+
| A | Aarav |
| B | Mohan |
| C | Jai |
| D | Harshit |
+----+---------+
4 rows in set (0.00 sec)现在，INNER JOIN 的查询可以如下 -
mysql> SELECT tbl_1.id,tbl_2.id FROM tbl_1 JOIN tbl_2 ON tbl_1.name = tbl_2.name;
+----+----+
| id | id |
+----+----+
| 4 | A |
+----+----+
1 row in set (0.00 sec)现在，如果我们在不指定条件的情况下编写上述查询，那么它将变成 CROSS JOIN。无条件查询如下 -
mysql> Select tbl_1.id, tbl_2.id FROM tbl_1 JOIN tbl_2;
+----+----+
| id | id |
+----+----+
| 1 | A |
| 2 | A |
| 3 | A |
| 4 | A |
| 1 | B |
| 2 | B |
| 3 | B |
| 4 | B |
| 1 | C |
| 2 | C |
| 3 | C |
| 4 | C |
| 1 | D |
| 2 | D |
| 3 | D |
| 4 | D |
+----+----+
16 rows in set (0.00 sec)
