Using A Count And A Case Statement In One Query - sql

Question

I'm pretty much out of ideas on how to get this to work.I haven't really used SQL in several years so there's a lot I don't remember.
So here is what I would like to happen:
I return the rows where the Code field from table has the value 1208 AND estnumber = 1187216
Run a count on the selection, if 0 run a subquery
If >0 run a different subquery
I didn't get to the subquery part yet because I can't get this to work correctly at all. Right now I just want it to return text.
Here is the latest attempt, I'm actually using db2 but maybe we can ignore that for now and i'll work that part out later because it says the syntax isnt correct, but other validators disagree (if you dont know anything about db2 just use standard sql when giving advice)
SELECT
count(*) AS t
FROM
table
WHERE
(
ESTNUMBER = 1187216
AND CODE = 1208
)
AND CASE WHEN t = 0 THEN 'it is zero' ELSE 'it is not zero' END;

Answer 1

Are you trying to do something like this?
WITH c AS (
SELECT count(*) AS cnt
FROM table
WHERE ESTNUMBER = 1187216 AND CODE = 1208
)
SELECT s1.*
FROM subquery1 s1
WHERE (SELECT cnt FROM c) = 0
UNION ALL
SELECT s2.*
FROM subquery2 s2
WHERE (SELECT cnt FROM c) > 0;
This assumes that the columns returned by the subqueries are compatible (same number, same types).
There are better ways to write this query (notably using EXISTS and NOT EXISTS), but this conforms directly to how you asked the question.

Answer 2

The string value should come up in the select clause and not in the where filter.
SELECT
count(*) AS t,
(CASE WHEN count(*) = 0 THEN 'it is zero' ELSE 'it is not zero' END) display_str
FROM
table
WHERE
(
ESTNUMBER = 1187216
AND CODE = 1208
)

Answer 3

You're thinking like an imperative programmer, not a declarative one. That is, SQL doesn't have sequential execution: it's all or nothing.
So, here's the start, the bit that works:
SELECT count(*) AS t
FROM table
WHERE ESTNUMBER = 1187216 AND CODE = 1208
Now, to check for the value of count(*), you by now know that WHERE isn't going to work. That's because COUNT is an aggregate function. To look at the result of such of function, you use HAVING.
For your CASE to work, you can move it up into the area that can get count(*) results:
SELECT count(*) AS t
(CASE WHEN count(*) = 0 THEN 'it is zero' ELSE 'it is not zero' END) as msg
FROM table
WHERE ESTNUMBER = 1187216 AND CODE = 1208
Note that "t" is an alias you've given the result of count(*). In most SQL implementations, that alias can't be leveraged in the rest of the statement.
Now, for the either or kind of thing, it would be time to reconsider your approach and what you're really after. You'll probably ultimately have both result sets in your statement and choose how the results are served up.
Something like:
select a.id, a.ct, (case when a.ct=0 then b.amt else c.amt end) as amt
from (select id, count(*) as ct from table1) a
left join (select id, sum(amount) as amt from table2) b on a.id=b.id
left join (select id, sum(amount) as amt from table3) c on a.id=c.id
Hope this helps.