从多个表中选择count（*）

如何count(*)从具有结果的两个不同表（分别称为tab1和tab2）中进行选择：

Count_1   Count_2
123       456

我已经试过了：

select count(*) Count_1 from schema.tab1 union all select count(*) Count_2 from schema.tab2

但是我所拥有的是：

Count_1
123
456

SELECT  (
        SELECT COUNT(*)
        FROM   tab1
        ) AS count1,
        (
        SELECT COUNT(*)
        FROM   tab2
        ) AS count2
FROM    dual

作为附加信息，要在SQL Server中完成相同的操作，您只需删除查询的“ FROM dual”部分。

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

它给出了换位的答案（每张表一行而不是一列），否则我认为这没有太大不同。我认为在性能方面，它们应该是等效的。

我的经验是使用SQL Server，但您可以这样做：

select (select count(*) from table1) as count1,
  (select count(*) from table2) as count2

在SQL Server中，我得到的结果是您追求的。

其他略有不同的方法：

with t1_count as (select count(*) c1 from t1),
     t2_count as (select count(*) c2 from t2)
select c1,
       c2
from   t1_count,
       t2_count
/

select c1,
       c2
from   (select count(*) c1 from t1) t1_count,
       (select count(*) c2 from t2) t2_count
/

我看不到任何其他答案。

如果您不喜欢子查询并且在每个表中都有主键，则可以执行以下操作：

select count(distinct tab1.id) as count_t1,
       count(distinct tab2.id) as count_t2
    from tab1, tab2

但是从性能角度来看，我相信Quassnoi的解决方案更好，而且我会使用。

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

这是我分享的

选项1-从不同表中的同一域计数

select distinct(select count(*) from domain1.table1) "count1", (select count(*) from domain1.table2) "count2" 
from domain1.table1, domain1.table2;

选项2-从同一表的不同域计数

select distinct(select count(*) from domain1.table1) "count1", (select count(*) from domain2.table1) "count2" 
from domain1.table1, domain2.table1;

选项3-使用“全部合并”从同一表的不同域进行计数以具有计数行

select 'domain 1'"domain", count(*) 
from domain1.table1 
union all 
select 'domain 2', count(*) 
from domain2.table1;

享受SQL，我总是做:)

    select 
    t1.Count_1,t2.Count_2
    from 
(SELECT count(1) as Count_1 FROM tab1) as t1, 
(SELECT count(1) as Count_2 FROM tab2) as t2

select (select count(*) from tab1) count_1, (select count(*) from tab2) count_2 from dual;

很快就想到了：

Select (select count(*) from Table1) as Count1, (select count(*) from Table2) as Count2

注意：我在SQL Server中对此进行了测试，因此From Dual没有必要（因此存在差异）。

出于完整性考虑，该查询将创建一个查询，以向您提供给定所有者的所有表的计数。

select 
  DECODE(rownum, 1, '', ' UNION ALL ') || 
  'SELECT ''' || table_name || ''' AS TABLE_NAME, COUNT(*) ' ||
  ' FROM ' || table_name  as query_string 
 from all_tables 
where owner = :owner;

输出是这样的

SELECT 'TAB1' AS TABLE_NAME, COUNT(*) FROM TAB1
 UNION ALL SELECT 'TAB2' AS TABLE_NAME, COUNT(*) FROM TAB2
 UNION ALL SELECT 'TAB3' AS TABLE_NAME, COUNT(*) FROM TAB3
 UNION ALL SELECT 'TAB4' AS TABLE_NAME, COUNT(*) FROM TAB4

然后可以运行以获取计数。有时只是一个方便的脚本。

如果表（或至少一个键列）的类型相同，则首先进行并集，然后进行计数。

select count(*) 
  from (select tab1key as key from schema.tab1 
        union all 
        select tab2key as key from schema.tab2
       )

或坐下，再加上一个sum（）。

select sum(amount) from
(
select count(*) amount from schema.tab1 union all select count(*) amount from schema.tab2
)

--============= 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

Declare @all int
SET @all = (select COUNT(*) from tab1) + (select count(*) from tab2)
Print @all

要么

SELECT (select COUNT(*) from tab1) + (select count(*) from tab2)

加入不同的表

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

选择（从tab1中field选择count（），如“值”）+（从tab2中选择count（），field如“值”）

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