粗心了，少打了 select

第一种

SELECT (count(*) FROM `bbs_thread` WHERE uid = 7) AS mythreads, (COUNT(*) FROM bbs_post WHERE uid = 7) AS myposts

第二种

SELECT (count(*) FROM `bbs_thread` WHERE uid = 7) AS mythreads, COUNT(*) AS myposts FROM bbs_post WHERE uid = 7

以上两种写法均不可行。

补充一下，是在 mysql 5.7.9 下