SQL CROSS JOIN 操作

在本教程中，您将学习如何使用 SQL 交叉连接从两个表中获取数据。

使用交叉连接

如果在连接两个表时不指定连接条件，数据库系统会将第一个表中的每一行与第二个表中的每一行组合起来。 这种类型的连接称为交叉连接或笛卡尔积。 下面的维恩图说明了交叉连接的工作原理。

为了容易理解这一点，让我们看看下面的 employees 和 departments 表。

+--------+--------------+------------+---------+
| emp_id | emp_name     | hire_date  | dept_id |
+--------+--------------+------------+---------+
|      1 | Ethan Hunt   | 2001-05-01 |       4 |
|      2 | Tony Montana | 2002-07-15 |       1 |
|      3 | Sarah Connor | 2005-10-18 |       5 |
|      4 | Rick Deckard | 2007-01-03 |       3 |
|      5 | Martin Blank | 2008-06-24 |    NULL |
+--------+--------------+------------+---------+

+---------+------------------+
| dept_id | dept_name        |
+---------+------------------+
|       1 | Administration   |
|       2 | Customer Service |
|       3 | Finance          |
|       4 | Human Resources  |
|       5 | Sales            |
+---------+------------------+

交叉连接中的行数是每个表中行数的乘积。 这是一个简单的交叉连接操作示例。

SELECT t1.emp_id, t1.emp_name, t1.hire_date, t2.dept_name
FROM employees AS t1 CROSS JOIN departments AS t2;

执行上述命令后，您会得到如下结果集：

+--------+--------------+------------+------------------+
| emp_id | emp_name     | hire_date  | dept_name        |
+--------+--------------+------------+------------------+
|      1 | Ethan Hunt   | 2001-05-01 | Administration   |
|      2 | Tony Montana | 2002-07-15 | Administration   |
|      3 | Sarah Connor | 2005-10-18 | Administration   |
|      4 | Rick Deckard | 2007-01-03 | Administration   |
|      5 | Martin Blank | 2008-06-24 | Administration   |
|      1 | Ethan Hunt   | 2001-05-01 | Customer Service |
|      2 | Tony Montana | 2002-07-15 | Customer Service |
|      3 | Sarah Connor | 2005-10-18 | Customer Service |
|      4 | Rick Deckard | 2007-01-03 | Customer Service |
|      5 | Martin Blank | 2008-06-24 | Customer Service |
|      1 | Ethan Hunt   | 2001-05-01 | Finance          |
|      2 | Tony Montana | 2002-07-15 | Finance          |
|      3 | Sarah Connor | 2005-10-18 | Finance          |
|      4 | Rick Deckard | 2007-01-03 | Finance          |
|      5 | Martin Blank | 2008-06-24 | Finance          |
|      1 | Ethan Hunt   | 2001-05-01 | Human Resources  |
|      2 | Tony Montana | 2002-07-15 | Human Resources  |
|      3 | Sarah Connor | 2005-10-18 | Human Resources  |
|      4 | Rick Deckard | 2007-01-03 | Human Resources  |
|      5 | Martin Blank | 2008-06-24 | Human Resources  |
|      1 | Ethan Hunt   | 2001-05-01 | Sales            |
|      2 | Tony Montana | 2002-07-15 | Sales            |
|      3 | Sarah Connor | 2005-10-18 | Sales            |
|      4 | Rick Deckard | 2007-01-03 | Sales            |
|      5 | Martin Blank | 2008-06-24 | Sales            |
+--------+--------------+------------+------------------+

如您所见，交叉连接不如我们在前几章中介绍的其他连接有用。 由于查询没有指定连接条件，因此employees 表中的每一行都与departments 表中的每一行相结合。 因此，除非您确定要使用笛卡尔积，否则不要使用交叉连接。