在SQL中，JOIN和CROSS JOIN有什么区别? [英] In SQL, what's the difference between JOIN and CROSS JOIN?

问题描述

之间有什么区别

select t1.a1, t1.a2, t1.a3 from t1 cross join t2 where t1.a3 = t2.a1

和:

select t1.a1, t1.a2, t1.a3 from t1,t2 where t1.a3=t2.a1;

我可以互换使用吗?

推荐答案

MySQL在JOIN和CROSS JOIN之间没有区别.他们是一样的.

MySQL doesn't offer a distinction between JOIN and CROSS JOIN. They are the same.

在两个示例中，子句

WHERE t1.a3 = t2.a1 

将任何类型的联接转换为内部联接.表达此查询的标准方式是

converts any sort of join into an inner join. The standard way of expressing this query is

SELECT t1.a1, t1.a2, t1.a3 
  FROM t1 
  JOIN t2 ON t1.a3 = t2.a1

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