人生苦短，务必性感。

记得很早前第一次接触mysql时，就想多表查个东西，没想到报错了~

想着以前用oracle也是这样查的呀，真是百思不得其解...

难道是不同sql语句有差异？？？

算了，google大法好！！！

每个派生表都必须有自己的别名

直白地说，加个别名就行了，就是这个意思！！！

附上语句

报错SQL语句：

SELECT SUM(b15) FROM(
(SELECT COUNT(1) AS b15 FROM usmschis.`t_his_telemeterdata20201105` WHERE lsc_id = '18')
UNION
(SELECT COUNT(1) AS b15 FROM usmschis.`t_his_telesignaldata20201105` WHERE lsc_id = '18')
) ;

正确SQL语句：

SELECT SUM(b15) FROM(
(SELECT COUNT(1) AS b15 FROM usmschis.`t_his_telemeterdata20201105` WHERE lsc_id = '18')
UNION
(SELECT COUNT(1) AS b15 FROM usmschis.`t_his_telesignaldata20201105` WHERE lsc_id = '18')
) AS sum_all;

真水，舒服

最后，少用union，哈哈哈