How to use string as column name in Bigquery - google-cloud-platform

Question

There is a scenario where I receive a string to the bigquery function and need to use it as a column name.
here is the function
CREATE OR REPLACE FUNCTION METADATA.GET_VALUE(column STRING, row_number int64) AS (
(SELECT column from WORK.temp WHERE rownumber = row_number)
);
When I call this function as select METADATA.GET_VALUE("TXCAMP10",149); I get the value as TXCAMP10 so we can say that it is processed as SELECT "TXCAMP10" from WORK.temp WHERE rownumber = 149 but I need it as SELECT TXCAMP10 from WORK.temp WHERE rownumber = 149 which will return some value from temp table lets suppose the value as A
so ultimately I need value A instead of column name i.e. TXCAMP10.
I tried using execute immediate like execute immediate("SELECT" || column || "from WORK.temp WHERE rownumber =" ||row_number) from this stack overflow post to resolve this issue but turns out I can't use it in a function.
How do I achieve required result?

Answer 1

I don't think you can achieve this result with the help of UDF in standard SQL in BigQuery.
But it is possible to do this with stored procedures in BigQuery and EXECUTE IMMEDIATE statement. Consider this code, which simulates the situation you have:
create or replace table d1.temp(
c1 int64,
c2 int64
);
insert into d1.temp values (1, 1), (2, 2);
create or replace procedure d1.GET_VALUE(column STRING, row_number int64, out result int64)
BEGIN
EXECUTE IMMEDIATE 'SELECT ' || column || ' from d1.temp where c2 = ?' into result using row_number;
END;
BEGIN
DECLARE result_c1 INT64;
call d1.GET_VALUE("c1", 1, result_c1);
select result_c1;
END;

Answer 2

After some research and trial-error methods, I used this workaround to solve this issue. It may not be the best solution when you have too many columns but it surely works.
CREATE OR REPLACE FUNCTION METADATA.GET_VALUE(column STRING, row_number int64) AS (
(SELECT case
when column_name = 'a' then a
when column_name = 'b' then b
when column_name = 'c' then c
when column_name = 'd' then d
when column_name = 'e' then e
end from WORK.temp WHERE rownumber = row_number)
);
And this gives the required results.
Point to note: the number of columns you use in the case statement should be of the same datatype else it won't work