SELECT (SELECT COUNT(*) FROM table1) + (SELECT COUNT(*) FROM table2) FROM dual;

与不同的表进行JOIN

SELECT COUNT(*) FROM (  
SELECT DISTINCT table_a.ID  FROM table_a JOIN table_c ON table_a.ID  = table_c.ID   );

--============= FIRST WAY (Shows as Multiple Row) ===============
SELECT 'tblProducts' [TableName], COUNT(P.Id) [RowCount] FROM tblProducts P
UNION ALL
SELECT 'tblProductSales' [TableName], COUNT(S.Id) [RowCount] FROM tblProductSales S


--============== SECOND WAY (Shows in a Single Row) =============
SELECT  
(SELECT COUNT(Id) FROM   tblProducts) AS ProductCount,
(SELECT COUNT(Id) FROM   tblProductSales) AS SalesCount

select @count = sum(data) from
(
select count(*)  as data from #tempregion
union 
select count(*)  as data from #tempmetro
union
select count(*)  as data from #tempcity
union
select count(*)  as data from #tempzips
) a

只是因为它略有不同:

SELECT 'table_1' AS table_name, COUNT(*) FROM table_1
UNION
SELECT 'table_2' AS table_name, COUNT(*) FROM table_2
UNION
SELECT 'table_3' AS table_name, COUNT(*) FROM table_3

它给出了转置的答案(每个表一行而不是一列)，否则我不认为它有多大不同。我认为在性能方面，它们应该是相等的。

SELECT  (
        SELECT COUNT(*)
        FROM   tbl1
        )
        +
        (
        SELECT COUNT(*)
        FROM   tbl2
        ) 
    as TotalCount