SQL Server：一次結合COUNT（*）爲不同表格

Question

我有幾個查詢，我想合併到一個查詢，以便不必多次呼叫到服務器。

我使用的查詢的一個例子：

SELECT COUNT(*) AS mailCount1 
FROM [WebContact].[dbo].[memberEmails] 
WHERE contactdatetime > '01/01/06' 
    AND contactdatetime < '02/01/06' 

SELECT COUNT(*) AS mailCount2 
FROM [WebContact].[dbo].[otherEmails] 
WHERE contactdatetime > '01/01/06' 
    AND contactdatetime < '02/01/06' 

SELECT COUNT(*) AS mailCount3 
FROM [WebContact].[dbo].[memberEmails] 
WHERE contactdatetime > '02/01/06' 
    AND contactdatetime < '03/01/06' 

SELECT COUNT(*) AS mailCount4 
FROM [WebContact].[dbo].[otherEmails] 
WHERE contactdatetime > '02/01/06' 
    AND contactdatetime < '03/01/06' 

etc etc... 

所以上面的例子，唯一的改變是：

• 的FROM（memberEmails & otherEmails）

• The > & <個月（01/01/06，02/01/06 | 02/01/06，03/01/06 |等等......）

這是可能做一個單一的查詢嗎？

Answer 1

首先，使用group by，只使用兩個查詢：

select year(contactdatetime) as yyyy, month(contactdatetime) as mm, count(*) 
from WebContact].[dbo].[memberEmails] 
group by year(contactdatetime), month(contactdatetime); 

和：

select year(contactdatetime) as yyyy, month(contactdatetime) as mm, count(*) 
from WebContact].[dbo].[otherEmails] 
group by year(contactdatetime), month(contactdatetime); 

然後，如果你願意，你可以將其組合到一個查詢：

select coalesce(me.yyyy, oe.yyyy) as yyyy, coalesce(me.mm, oe.mm) as mm, 
     coalesce(me.cnt, 0) as memberemailcnt, 
     coalesce(oe.cnt, 0) as otheremailcnt 
from (select year(contactdatetime) as yyyy, month(contactdatetime) as mm, count(*) as cnt 
     from WebContact].[dbo].[memberEmails] 
     group by year(contactdatetime), month(contactdatetime) 
    ) me full outer join 
    (select year(contactdatetime) as yyyy, month(contactdatetime) as mm, count(*) as cnt 
     from WebContact].[dbo].[otherEmails] 
     group by year(contactdatetime), month(contactdatetime) 
    ) oe 
    on me.yyyy = oe.yyyy and me.mm = oe.mm; 

如果兩個表都包含所有月份的數據，則不需要。

Answer 2

我沒有檢查語法或性能，但你可以做這樣的事情，

 WITH cte (
     countvalue 
     ,description 
     ) 
    AS (
     SELECT COUNT(*) 
      ,'mailCount1' 
     FROM [WebContact].[dbo].[memberEmails] 
     WHERE contactdatetime > '01/01/06' 
      AND contactdatetime < '02/01/06' 

     UNION ALL 

     SELECT COUNT(*) 
      ,'mailCount2' 
     FROM [WebContact].[dbo].[otherEmails] 
     WHERE contactdatetime > '01/01/06' 
      AND contactdatetime < '02/01/06' 

     UNION ALL 

     SELECT COUNT(*) 
      ,'mailCount3' 
     FROM [WebContact].[dbo].[memberEmails] 
     WHERE contactdatetime > '02/01/06' 
      AND contactdatetime < '03/01/06' 

     UNION ALL 

     SELECT COUNT(*) 
      ,'mailCount4' 
     FROM [WebContact].[dbo].[otherEmails] 
     WHERE contactdatetime > '02/01/06' 
      AND contactdatetime < '03/01/06' 
     ) 

    SELECT mailCount1 
    ,mailCount2 
    ,mailCount3 
    ,mailCount4 
FROM (
    SELECT countvalue 
     ,description 
    FROM cte 
    ) d 
pivot(max(countvalue) FOR description IN (mailCount1, mailCount2, mailCount3, mailCount4)) piv; 

希望這有助於..

Answer 3

declare @emailCount table(tablename varchar(20), year int, month int, qty int) 

insert into @emailCount 
select 'memberEmails', year(contactdatetime), month(contactdatetime), count(*) 
from [WebContact].[dbo].[memberEmails] 
group by year(contactdatetime), month(contactdatetime) 

insert into @emailCount 
select 'otherEmails',year(contactdatetime), month(contactdatetime), count(*) 
from [WebContact].[dbo].[otherEmails] 
group by year(contactdatetime), month(contactdatetime) 

select tablename, year, month, qty from @emailCount 

添加WHERE條款，如果需要限制日期範圍。 （編輯 - 簡化爲使用年份（）和月份（）函數。）