sql计数总和

Question

我有两个表,并希望每个表中的总行数.实际查询更复杂,因为每个计数都有...子句

如何在t-sql中执行以下操作(两者都不起作用)？

select count(*) from table1 + count(*) from table2

要么

select sum(count(*) from table1,count(*) from table2)

Answer 1

select SUM(cnt) from
(
    select COUNT(*) as cnt from table1 where /* where conditions */
    union all
    select COUNT(*) from table2 where /* where conditions */
) t

似乎可以解决这个问题,将不同表的查询分开,并轻松扩展到更多表.

Answer 2

select  (select count(PrimaryKeyID) from  FirstTable) 
      + (select COUNT(PrimaryKeyID) from TableSecond)

所以我认为我们应该避免在下面的查询中使用星号.因为它可能会导致查询性能下降

select  (select count(*) from  FirstTable) 
      + (select COUNT(*) from TableSecond)