SQL - where 子句中的连接字符串未按预期工作 - Redshift

Question

This is the SQL (Redshift) I am referring to:

SELECT * 
  FROM table_a
 WHERE col_a||col_b NOT IN 
            (
                SELECT col_a||col_b 
                  from table_b
            );

There are values in table_a which don't exist in table_b, yet this always evaluates to a No rows.

Any insight?

Answer 1

Use NOT EXISTS , not NOT IN !!! NOT IN will return no rows at all if any value in the subquery is NULL . Plus, you are comparing multiple columns by concatenating them. That is likely to be a bad method because of collisions: ab/c matches a/bc.

So use this:

SELECT a.* 
FROM table_a a
WHERE NOT EXISTS (SELECT 1
                  FROM table_b b
                  WHERE b.col_a = a.col_a AND b.col_b = a.col_b
                 );

Answer 2

The sub query might be having a null value, which is failing the not in clause.

Use coalesce to replace null with an appropriate value based on your data. I have used a ~ character in place of null

SELECT * 
  FROM table_a
 WHERE col_a||col_b NOT IN 
            (
                SELECT coalesce (col_a,'~')||coalesce (col_b , '~')
                  from table_b
            );