SQL Server: How to use UNION with two queries that BOTH have a WHERE clause?

Question

Given:

Two queries that require filtering:

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


        

Answer 1

Create views on two first "selects" and "union" them.

Answer 2

The answer is misleading because it attempts to fix a problem that is not a problem. You actually CAN have a WHERE CLAUSE in each segment of a UNION. You cannot have an ORDER BY except in the last segment. Therefore, this should work...

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

Answer 3

The basic premise of the question and the answers are wrong. Every Select in a union can have a where clause. It's the ORDER BY in the first query that's giving yo the error.

Answer 4

You should be able to alias them and use as subqueries (part of the reason your first effort was invalid was because the first select had two columns (ID and ReceivedDate) but your second only had one (ID) - also, Type is a reserved word in SQL Server, and can't be used as you had it as a column name):

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 5

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

or using CTE (SQL Server 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 6

Notice that each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order. you are selecting

t1.ID, t2.ReceivedDate from Table t1

union

t2.ID from Table t2

which is incorrect.

so you have to write

t1.ID, t1.ReceivedDate from Table t1 union t2.ID, t2.ReceivedDate from Table t1

you can use sub query here

 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 

so it will return only distinct values by default from both table.