LEFT JOIN praise as a on a.userid=i.userid
WHERE i.`group`=101 and(a.tid is null or a.tid !=20);
Analyze TableMySQL的Optimizer(優化元件)在優化SQL語句時,首先需要收集一些相關信息,其中就包括表的cardinality
select tbs as 表空間名稱, sum(totalM) as 空間合計M, sum(usedM) as 已使用空間, sum(remainedM) as 剩餘空間M, su