JOIN 和 UNION 有什么区别? [英] What is the difference between JOIN and UNION?

问题描述

JOIN 和 UNION 有什么区别?可以举个例子吗?

What is the difference between JOIN and UNION? Can I have an example?

推荐答案

UNION 将查询中的行放在一起，而 JOIN 生成笛卡尔积并将其子集 -- 完全不同的操作.UNION 的简单例子:

UNION puts lines from queries after each other, while JOIN makes a cartesian product and subsets it -- completely different operations. Trivial example of UNION:

mysql> SELECT 23 AS bah
    -> UNION
    -> SELECT 45 AS bah;
+-----+
| bah |
+-----+
|  23 | 
|  45 | 
+-----+
2 rows in set (0.00 sec)

JOIN 类似的简单例子:

mysql> SELECT * FROM 
    -> (SELECT 23 AS bah) AS foo 
    -> JOIN 
    -> (SELECT 45 AS bah) AS bar
    -> ON (33=33);
+-----+-----+
| foo | bar |
+-----+-----+
|  23 |  45 | 
+-----+-----+
1 row in set (0.01 sec)

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