Hello @Lily99 , I hope this message finds you well.

Could you please try the code below and let me know the results?

CREATE OR REPLACE FUNCTION fn_abc(var1 STRING, var2 STRING)

RETURNS DOUBLE
COMMENT 'test function'
RETURN SELECT
    CASE
    WHEN EXISTS(
        SELECT COUNT(1)
        FROM tablea f INNER JOIN tableb t WHERE lower(f.col1) = t.col1)
    THEN 10.5
    END;

Best regards,
Lucas Rocha