How do I combine 2 select statements into one?

Solution 1:

You have two choices here. The first is to have two result sets which will set 'Test1' or 'Test2' based on the condition in the WHERE clause, and then UNION them together:

select 
    'Test1', * 
from 
    TABLE 
Where 
    CCC='D' AND DDD='X' AND exists(select ...)
UNION
select 
    'Test2', * 
from 
    TABLE
Where
    CCC<>'D' AND DDD='X' AND exists(select ...)

This might be an issue, because you are going to effectively scan/seek on TABLE twice.

The other solution would be to select from the table once, and set 'Test1' or 'Test2' based on the conditions in TABLE:

select 
    case 
        when CCC='D' AND DDD='X' AND exists(select ...) then 'Test1'
        when CCC<>'D' AND DDD='X' AND exists(select ...) then 'Test2'
    end,
    * 
from 
    TABLE 
Where 
    (CCC='D' AND DDD='X' AND exists(select ...)) or
    (CCC<>'D' AND DDD='X' AND exists(select ...))

The catch here being that you will have to duplicate the filter conditions in the CASE statement and the WHERE statement.

Solution 2:

If they are from the same table, I think UNION is the command you're looking for.

(If you'd ever need to select values from columns of different tables, you should look at JOIN instead...)

Solution 3:

Thanks for the input. Tried the stuff that has been mentioned here and these are the 2 I got to work:

(
select 'OK', * from WorkItems t1
where exists(select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1) )
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00'
AND (BoolField05=1)
)
UNION
(
select 'DEL', * from WorkItems t1
where exists(select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1) )
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00'
AND NOT (BoolField05=1)
)

AND

select 
    case
        when
            (BoolField05=1)
    then 'OK'
    else 'DEL'
        end,
        *
from WorkItems t1
Where
            exists(select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1) )
            AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
            AND TimeStamp>'2009-02-12 18:00:00'

Which would be the most efficient of these (edit: the second as it only scans the table once), and is it possible to make it even more efficient? (The BoolField=1) is really a variable (dyn sql) that can contain any where statement on the table.

I am running on MS SQL 2005. Tried Quassnoi examples but did not work as expected.

Solution 4:

select t1.* from 
(select * from TABLE Where CCC='D' AND DDD='X') as t1,
(select * from TABLE Where CCC<>'D' AND DDD='X') as t2

Another way to do this!

Solution 5:

select Status, * from WorkItems t1
where  exists (select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1) )
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00'

UNION

select 'DELETED', * from WorkItems t1
where  exists (select 1 from workitems t2 where t1.TextField01=t2.TextField01 AND (BoolField05=1) )
AND TimeStamp=(select max(t2.TimeStamp) from workitems t2 where t2.TextField01=t1.TextField01) 
AND TimeStamp>'2009-02-12 18:00:00'
AND NOT (BoolField05=1)

Perhaps that'd do the trick. I can't test it from here though, and I'm not sure what version of SQL you're working against.