mysql子查询是什么

#外查询

SELECT "栏位1" FROM "表格1" WHERE "栏位2" [比较运算符]

#内查询

(SELECT "栏位1" FROM "表格2" WHERE "条件");

#可以是符号的运算符，例如：=、>、<、>=、<= ；也可以是文字的运算符，例如 LIKE、IN、BETWEEN

select sum(sales) from info where place_name in (select place_name from destination where region = 'southwest');

#下面这个句子就是上面的简化版

select sum(sales) from info where place_name in('chengdu','kunming');