SQL Server：如何使用UNION和兩個查詢都有WHERE子句？

Question

考慮：

select top 2 t1.ID, t1.ReceivedDate 
    from Table t1 
where t1.Type = 'TYPE_1' 
order by t1.ReceivedDate desc 

而且：需要過濾

兩個查詢

select top 2 t2.ID 
    from Table t2 
where t2.Type = 'TYPE_2' 
order by t2.ReceivedDate desc 

另外，這些返回ID了，我就是在尋找：（13，11和12，6）

基本上，我想要兩個特定的最近兩個記錄數據的pes。

我想聯盟這兩個查詢等一起這樣：

select top 2 t1.ID, t2.ReceivedDate 
    from Table t1 
where t1.Type = 'TYPE_1' 
order by ReceivedDate desc 
union 
select top 2 t2.ID 
    from Table t2 
where t2.Type = 'TYPE_2' 
order by ReceivedDate desc 

問題：

的問題是，這種查詢是無效的，因爲第一select不能有order by條款，如果它是正在unioned。如果沒有order by，它不能有top 2。

我該如何解決這種情況？

Answer 1

你應該能夠別名他們和子查詢使用你的第一個努力是無效的原因（部分是因爲第一個選擇有兩列（ID和ReceivedDate），但你只有一秒鐘（ID） - 此外，類型是SQL Server中的保留字，不能用作您把它作爲一個列名）：

declare @Tbl1 table(ID int, ReceivedDate datetime, ItemType Varchar(10)) 
declare @Tbl2 table(ID int, ReceivedDate datetime, ItemType Varchar(10)) 

insert into @Tbl1 values(1, '20010101', 'Type_1') 
insert into @Tbl1 values(2, '20010102', 'Type_1') 
insert into @Tbl1 values(3, '20010103', 'Type_3') 

insert into @Tbl2 values(10, '20010101', 'Type_2') 
insert into @Tbl2 values(20, '20010102', 'Type_3') 
insert into @Tbl2 values(30, '20010103', 'Type_2') 

SELECT a.ID, a.ReceivedDate FROM 
(select top 2 t1.ID, t1.ReceivedDate 
    from @tbl1 t1 
    where t1.ItemType = 'TYPE_1' 
    order by ReceivedDate desc 
) a 
union 
SELECT b.ID, b.ReceivedDate FROM 
(select top 2 t2.ID, t2.ReceivedDate 
    from @tbl2 t2 
    where t2.ItemType = 'TYPE_2' 
    order by t2.ReceivedDate desc 
) b 

Answer 2

創建兩個第一個「選擇」和「聯合」他們的意見。

Answer 3

declare @T1 table(ID int, ReceivedDate datetime, [type] varchar(10)) 
declare @T2 table(ID int, ReceivedDate datetime, [type] varchar(10)) 

insert into @T1 values(1, '20010101', '1') 
insert into @T1 values(2, '20010102', '1') 
insert into @T1 values(3, '20010103', '1') 

insert into @T2 values(10, '20010101', '2') 
insert into @T2 values(20, '20010102', '2') 
insert into @T2 values(30, '20010103', '2') 

;with cte1 as 
(
    select *, 
    row_number() over(order by ReceivedDate desc) as rn 
    from @T1 
    where [type] = '1' 
), 
cte2 as 
(
    select *, 
    row_number() over(order by ReceivedDate desc) as rn 
    from @T2 
    where [type] = '2' 
) 
select * 
from cte1 
where rn <= 2 
union all 
select * 
from cte2 
where rn <= 2 

Answer 4

select * from 
(
    select top 2 t1.ID, t1.ReceivedDate 
    from Table t1 
    where t1.Type = 'TYPE_1' 
    order by t1.ReceivedDate de 
) t1 
union 
select * from 
(
    select top 2 t2.ID 
    from Table t2 
    where t2.Type = 'TYPE_2' 
    order by t2.ReceivedDate desc 
) t2 

或使用CTE（SQL服務器2005+）

;with One as 
(
    select top 2 t1.ID, t1.ReceivedDate 
    from Table t1 
    where t1.Type = 'TYPE_1' 
    order by t1.ReceivedDate de 
) 
,Two as 
(
    select top 2 t2.ID 
    from Table t2 
    where t2.Type = 'TYPE_2' 
    order by t2.ReceivedDate desc 
) 
select * from One 
union 
select * from Two 

Answer 5

問題和答案的基本前提是錯誤的。在聯合中的每個選擇都可以有一個where子句。這是ORDER BY在第一個查詢中給出的錯誤。

Answer 6

請注意，UNION中的每個SELECT語句必須具有相同的列數。這些列也必須具有相似的數據類型。另外，每個SELECT語句中的列必須以相同的順序排列。 您選擇

t1.ID，t2.ReceivedDate 從表T1

工會

從表T2

t2.ID

這是不正確。

，所以你必須寫

t1.ID，t1.ReceivedDate從表T1 工會 t2.ID，t2.ReceivedDate從表T1

，你可以在這裏使用子查詢

SELECT tbl1.ID, tbl1.ReceivedDate FROM 
     (select top 2 t1.ID, t1.ReceivedDate 
     from tbl1 t1 
     where t1.ItemType = 'TYPE_1' 
     order by ReceivedDate desc 
    ) tbl1 
union 
    SELECT tbl2.ID, tbl2.ReceivedDate FROM 
    (select top 2 t2.ID, t2.ReceivedDate 
     from tbl2 t2 
     where t2.ItemType = 'TYPE_2' 
     order by t2.ReceivedDate desc 
    ) tbl2 

因此，它只會從這兩個表中默認返回不同的值。

Answer 7

答案是誤導性的，因爲它試圖解決一個不是問題的問題。你實際上可以在UNION的每個段中都有一個WHERE CLAUSE。除最後一段以外，您不能擁有ORDER BY。因此，這應該工作...

select top 2 t1.ID, t1.ReceivedDate 
from Table t1 
where t1.Type = 'TYPE_1' 
-----remove this-- order by ReceivedDate desc 
union 
select top 2 t2.ID, t2.ReceivedDate --- add second column 
    from Table t2 
where t2.Type = 'TYPE_2' 
order by ReceivedDate desc