从CTE中选择，可能为空/未定义

Question

Inside a function/stored procedure in Postgres 9.6 I want to grab data from two different tables using one CTE for each table like so:

WITH "CTE_from_table1" AS (SELECT column1, column2 FROM table1 WHERE id = $1),
     "CTE_from_table2" AS (SELECT column1, column2 FROM table2 WHERE id = $2)
SELECT 
  COALESCE(
     "CTE_from_table1".column1,
     "CTE_from_table2".column1,    
     CASE WHEN "CTE_from_table1".column2 = 42 OR "CTE_from_table2".column2 = 42 
          THEN 'Something is 42' ELSE 'something else!' END
   )
FROM "CTE_from_table1","CTE_from_table2";

(Data type of column1 and column2 are resp. identical for both tables: column1 being a text, column2 an integer.)

That works as long as both CTEs are defined. The problem is: The parameters $1 and/or $2 could be null or could contain IDs which are simply not there. In this case, I expect the result something else! , because the first two COALESCE parameters evaluate to null and the third, being the CASE WHEN , should go to its ELSE which would return something else! .

That's my theory. However, in practice I get null as soon as one of the CTEs is undefined/null. What can I do about that?

Answer 1

Your problem is the dreaded comma in the FROM clause. Simple rule . . . Never use commas in the FROM clause. In this case, you want an "outer cross join". The comma does an "inner cross join", so no rows are returned if either CTE has no rows.

Unfortunately, OUTER CROSS JOIN doesn't exist, so you can make do with FULL OUTER JOIN :

WITH "CTE_from_table1" AS (SELECT column1, column2 FROM table1 WHERE id = $1),
     "CTE_from_table2" AS (SELECT column1, column2 FROM table2 WHERE id = $2)
SELECT COALESCE(ct1.column1, ct2.column1,    
                CASE WHEN 42 IN (ct1.column2, ct2.column2)
                     THEN 'Something is 42'
                     ELSE 'something else!'
                END
               )
FROM "CTE_from_table1" ct1 FULL OUTER JOIN
     "CTE_from_table2" ct2
     ON 1=1;

I'm not a big fan of mixing CASE and COALESCE() , so I'd be inclined to write:

SELECT (CASE WHEN ct1.column1 IS NOT NULL THEN ct1.column1
             WHEN ct2.column1 IS NOT NULL THEN ct2.column1 
             WHEN 42 IN (ct1.column2, ct2.column2) THEN 'Something is 42'
             ELSE 'something else!'
        END)