Possible to use a column name in a UDF in SQL? - sql

Question

I have a query in which a series of steps is repeated constantly over different columns, for example:
SELECT DISTINCT
MAX (
CASE
WHEN table_2."GRP1_MINIMUM_DATE" <= cohort."ANCHOR_DATE" THEN 1
ELSE 0
END)
OVER (PARTITION BY cohort."USER_ID")
AS "GRP1_MINIMUM_DATE",
MAX (
CASE
WHEN table_2."GRP2_MINIMUM_DATE" <= cohort."ANCHOR_DATE" THEN 1
ELSE 0
END)
OVER (PARTITION BY cohort."USER_ID")
AS "GRP2_MINIMUM_DATE"
FROM INPUT_COHORT cohort
LEFT JOIN INVOLVE_EVER table_2 ON cohort."USER_ID" = table_2."USER_ID"
I was considering writing a function to accomplish this as doing so would save on space in my query. I have been reading a bit about UDF in SQL but don't yet understand if it is possible to pass a column name in as a parameter (i.e. simply switch out "GRP1_MINIMUM_DATE" for "GRP2_MINIMUM_DATE" etc.). What I would like is a query which looks like this
SELECT DISTINCT
FUNCTION(table_2."GRP1_MINIMUM_DATE") AS "GRP1_MINIMUM_DATE",
FUNCTION(table_2."GRP2_MINIMUM_DATE") AS "GRP2_MINIMUM_DATE",
FUNCTION(table_2."GRP3_MINIMUM_DATE") AS "GRP3_MINIMUM_DATE",
FUNCTION(table_2."GRP4_MINIMUM_DATE") AS "GRP4_MINIMUM_DATE"
FROM INPUT_COHORT cohort
LEFT JOIN INVOLVE_EVER table_2 ON cohort."USER_ID" = table_2."USER_ID"
Can anyone tell me if this is possible/point me to some resource that might help me out here?
Thanks!

Answer 1

There is no such direct as #Tejash already stated, but the thing looks like your database model is not ideal - it would be better to have a table that has USER_ID and GRP_ID as keys and then MINIMUM_DATE as seperate field.
Without changing the table structure, you can use UNPIVOT query to mimic this design:
WITH INVOLVE_EVER(USER_ID, GRP1_MINIMUM_DATE, GRP2_MINIMUM_DATE, GRP3_MINIMUM_DATE, GRP4_MINIMUM_DATE)
AS (SELECT 1, SYSDATE, SYSDATE, SYSDATE, SYSDATE FROM dual UNION ALL
SELECT 2, SYSDATE-1, SYSDATE-2, SYSDATE-3, SYSDATE-4 FROM dual)
SELECT *
FROM INVOLVE_EVER
unpivot ( minimum_date FOR grp_id IN ( GRP1_MINIMUM_DATE AS 1, GRP2_MINIMUM_DATE AS 2, GRP3_MINIMUM_DATE AS 3, GRP4_MINIMUM_DATE AS 4))
Result:
| USER_ID | GRP_ID | MINIMUM_DATE |
|---------|--------|--------------|
| 1 | 1 | 09/09/19 |
| 1 | 2 | 09/09/19 |
| 1 | 3 | 09/09/19 |
| 1 | 4 | 09/09/19 |
| 2 | 1 | 09/08/19 |
| 2 | 2 | 09/07/19 |
| 2 | 3 | 09/06/19 |
| 2 | 4 | 09/05/19 |
With this you can write your query without further code duplication and if you need use PIVOT-syntax to get one line per USER_ID.
The final query could then look like this:
WITH INVOLVE_EVER(USER_ID, GRP1_MINIMUM_DATE, GRP2_MINIMUM_DATE, GRP3_MINIMUM_DATE, GRP4_MINIMUM_DATE)
AS (SELECT 1, SYSDATE, SYSDATE, SYSDATE, SYSDATE FROM dual UNION ALL
SELECT 2, SYSDATE-1, SYSDATE-2, SYSDATE-3, SYSDATE-4 FROM dual)
, INPUT_COHORT(USER_ID, ANCHOR_DATE)
AS (SELECT 1, SYSDATE-1 FROM dual UNION ALL
SELECT 2, SYSDATE-2 FROM dual UNION ALL
SELECT 3, SYSDATE-3 FROM dual)
-- Above is sampledata query starts from here:
, unpiv AS (SELECT *
FROM INVOLVE_EVER
unpivot ( minimum_date FOR grp_id IN ( GRP1_MINIMUM_DATE AS 1, GRP2_MINIMUM_DATE AS 2, GRP3_MINIMUM_DATE AS 3, GRP4_MINIMUM_DATE AS 4)))
SELECT qcsj_c000000001000000 user_id, GRP1_MINIMUM_DATE, GRP2_MINIMUM_DATE, GRP3_MINIMUM_DATE, GRP4_MINIMUM_DATE
FROM INPUT_COHORT cohort
LEFT JOIN unpiv table_2
ON cohort.USER_ID = table_2.USER_ID
pivot (MAX(CASE WHEN minimum_date <= cohort."ANCHOR_DATE" THEN 1 ELSE 0 END) AS MINIMUM_DATE
FOR grp_id IN (1 AS GRP1,2 AS GRP2,3 AS GRP3,4 AS GRP4))
Result:
| USER_ID | GRP1_MINIMUM_DATE | GRP2_MINIMUM_DATE | GRP3_MINIMUM_DATE | GRP4_MINIMUM_DATE |
|---------|-------------------|-------------------|-------------------|-------------------|
| 3 | | | | |
| 1 | 0 | 0 | 0 | 0 |
| 2 | 0 | 1 | 1 | 1 |
This way you only have to write your calculation logic once (see line starting with pivot).