How to replace NULL value in select with subquery - sql

Question

Hi I'm performing a left join on two tables. If a particular column is NULL I want to run a subquery to get a value from a completely different table. Here's what I have now:
SELECT A.ACCOUNT_NUM, A.USER_ID,
CASE B.PREFERRED_NAME
WHEN '' THEN RTRIM(B.FIRST_NAME) || ' ' || B.LAST_NAME
ELSE RTRIM(B.PREFERRED_NAME) || ' ' || B.LAST_NAME
END AS NAME
FROM TABLE_A A
LEFT JOIN TABLE_B B
ON A.USER_ID = B.USER_ID
TABLE_B sometimes doesn't contain a record that matches with TABLE_A, so I want to run a subquery from TABLE_C that contains usernames and will match on A.USER_ID.
I thought I could do something like:
CASE B.PREFERRED_NAME
WHEN NULL THEN subquery here
But I get this error:
ERROR [42703] [IBM][DB2] SQL0206N "NULL" is not valid in the context where it is used.
Probably because NULLs are not allowed for that column.
SOLVED
Thanks for the help. This is how I solved my issue:
SELECT A.ACCOUNT_NUM, A.USER_ID,
CASE
WHEN B.PREFERRED_NAME IS NULL THEN C.USER_ID
WHEN B.PREFERRED_NAME IS NOT NULL THEN
CASE PREFERRED_NAME
WHEN '' THEN RTRIM(B.FIRST_NAME) || ' ' || B.LAST_NAME
ELSE RTRIM(B.PREFERRED_NAME) || ' ' || B.LAST_NAME
END
END AS NAME
FROM TABLE_A A
LEFT JOIN TABLE_B B
ON A.USER_ID = B.USER_ID
JOIN TABLE_C C
ON A.USER_ID = C.USER_ID

Answer 1

Depending on your query, you can probably just add your third table as another LEFT JOIN, then add the column you want to a COALESCE function:
Also, it looks like you're storing the preferred name as spaces if there isn't one, in which case you can use the NULLIF function to convert it to a NULL, which will work with your COALESCE.
Here's an example of what I mean:
SELECT
A.ACCOUNT_NUM
,A.USER_ID
,COALESCE(
NULLIF(B.PREFERRED_NAME,'')
,B.FIRST_NAME
,C.OTHER_NAME
) || ' ' || B.LAST_NAME AS NAME
FROM TABLE_A A
LEFT JOIN TABLE_C C
ON C.USER_ID = A.USER_ID
LEFT JOIN TABLE_B B
ON A.USER_ID = B.USER_ID
If you know there is always going to be a row in C that matches A, then you could convert that to a regular (inner) JOIN.
The reason you're getting the error, though is because you can't use NULL like that in a CASE statement. If you want to have a NULL case, then you have to do it like #Abecee said in the comment with CASE WHEN B.PREFERRED_NAME IS NULL THEN ...