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

问题描述

JOIN 和 UNION 之间有什么区别？我可以有个例子吗？

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

推荐答案

UNION 彼此，而 JOIN 使得一个笛卡尔产品和子集它 - 完全不同的操作。 UNION 的小事例子：

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 的类似示例：

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

这篇关于JOIN和UNION有什么区别？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！