SQL IF SELECT query is null then do another query - mysql

Question

I have a query that regularly returns "nothing", and I would like to run a different query if this is the case, but I know not of the way of doing this. If anyone could be of help please.
Here is the current code I am using...
SELECT * FROM cfg_users JOIN cfg_ash ON cfg_users.iUserId = cfg_ash.iUserid WHERE iTeamId='0' AND sDisabled IS NULL AND iStatusId > 0 AND sDate = '2014-08-01' GROUP BY cfg_users.iUserId ORDER BY iStatusId, sName
I basically want to say
IF <my code> IS NULL THEN <do other code>, IF <my code> IS NOT NULL THEN return the result.
Thanks

Answer 1

There are some simple way only use sql.
Define your first query as a temp table, with union all, filter the second query with temp table's count.
with temp as (select * from t1 where 1=0)
select * from temp
union all
select * from t2 where (select count(*) from temp) =0
This query will return the second table's records.
with temp as (select * from t1 )
select * from temp
union all
select * from t2 where (select count(*) from temp) =0
And if temp query have result, only return temp query.
You can test with sql fiddle here.

Answer 2

A way you can do it is like this
set two variables equal to the queries you want to execute.
set another variable equal to the correct query when the first is not null.
execute that query with a stored procedure.
STORED PROCEDURE:
DELIMITER $$
CREATE PROCEDURE `dynamic_query`(in input varchar(255))
BEGIN
SET #a := input;
PREPARE stmt FROM #a;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
END
$$
DELIMITER ;
THE TWO SELECTS YOU WANT TO EXECUTE:
SET #A := "SELECT * FROM cfg_users JOIN cfg_ash ON cfg_users.iUserId = cfg_ash.iUserid WHERE iTeamId='0' AND sDisabled IS NULL AND iStatusId > 0 AND sDate = '2014-08-01' GROUP BY cfg_users.iUserId ORDER BY iStatusId, sName";
SET #B := "your other select here";
THE DEFINER TO GET THE CORRECT QUERY:
SET #C := (
SELECT
CASE
WHEN EXISTS
( SELECT *
FROM cfg_users
JOIN cfg_ash ON cfg_users.iUserId = cfg_ash.iUserid
WHERE iTeamId='0'
AND sDisabled IS NULL
AND iStatusId > 0
AND sDate = '2014-08-01'
GROUP BY cfg_users.iUserId
ORDER BY iStatusId, sName
)
THEN #A
ELSE #B
END
);
EXECUTE THE STATEMENT:
CALL dynamic_query(#C);
DEMO WHEN THE QUERY EXISTS
DEMO WHEN THE QUERY DOESN'T EXIST

Answer 3

You can store the results in a temporary table / table variable, and then check the count
e.g.
CREATE TABLE #Results ( --columns you need here )
INSERT INTO #Results SELECT *
FROM cfg_users
JOIN cfg_ash ON cfg_users.iUserId = cfg_ash.iUserid WHERE iTeamId='0' AND sDisabled IS NULL AND iStatusId > 0 AND sDate = '2014-08-01'
GROUP BY cfg_users.iUserId
ORDER BY iStatusId, sName
SET #Count = SELECT COUNT(*) FROM #Results
IF 0 = #Count THEN
INSERT INTO #Results -- Other Query Here
SELECT * FROM #Results
n.b. you should really specify what columns you want in both queries rather than using *