如何在Mysql中计算Count（*）的总和

Question

我有两个表，我计算它们的行数。例如;

Select count(*) FROM tbl_Events

Select count(*) FROM tbl_Events2

我需要总数。如何用单一陈述对结果求和？

Answer 1

select sum(cnt) from (
    select count(*) as cnt from tbl_events
    union all
    select count(*) as cnt from tbl_events2
) as x

Answer 2

试试这个：

SELECT (Select count(*) FROM tbl_Events) + (Select count(*) FROM tbl_Events2)

或（在MSSQL中测试），这个：

SELECT COUNT(*) 
FROM (SELECT * FROM tbl_Events 
      UNION ALL 
      SELECT * FROM tbl_Events2) AS AllEvents

我猜第一个会带来更好的性能，因为它有更明显的索引选项。不过要测试一下。

Answer 3

Select Count(*)
From(
Select *    From tbl_Events
Union All
Select *    From tbl_Events2) as A