Question

say I have the following simple table

table a
{
id int,
pointId int,
state int,
value int
}

id    pointId     state   value
-------------------------------
1     1           2       xxxxx
2     3           2       xxxxx
3     3           2       xxxxx
4     3           1       xxxxx

with the following two queries:

SELECT * FROM a WHERE state = 1 and pointId = x
SELECT * FROM a WHERE state = 2 and pointId = x

Is there a way to combine the two queries so that:

• if query 1 returns >0 rows, it returns the result from query 1
• if query 1 returns 0 rows, it returns the result from query 2

(so for pointId = 1 it would return row 1, for pointId = 3 it would return row 4)

I’ve been trying to combine the two with a union doing something like

SELECT * FROM a WHERE state = 1 AND pointId = x
UNION ALL
SELECT * FROM a WHERE state = 2 AND pointId = x and 
pointId NOT IN (SELECT pointId FROM a WHERE state = 1 AND pointId = x)

I’m wondering if I’m making this way to complicated and if there’s a more easy way. I realize I could just run the first query and then handle it in my code, but it would be really nice if I could do this in the sql.
Any advice or pointers would be great!

Thanks,

A

Answer 1

UNION is used to combine tuples, your specification mentions you want to return 1 set of tuples or another set of tuples. In this case you don’t want to use UNION, you can use EXISTS:

IF EXISTS(...insert your first query here...)
    BEGIN
       --a record exists process first query
    END
ELSE
    BEGIN
         --do other query
    END