Question

I have two tables, Foo and Bar, I’d like to join them together so BarID gets the prize based on having a key that is in the array, or if there’s no array set up, it defaults as if every key unlocks the prize.

>Table Foo
fooID | someArray    | prize
----------------------------------
 1    | {10,20,30}   | 'Winner'
 2    | {10}         | 'Grand prize'
 3    | null         | 'Participant'

(That’s not a string for someArray, it’s a valid postgresql type.)

>Table Bar 
BarID | Key
------------
 1    | 10
 2    | 20
 3    | 30
 4    | 40
 5    | 40
 6    | 40

I’ve tried this below:

SELECT
    Foo.fooID,
    Bar.prize
FROM Foo
LEFT JOIN Bar ON Bar.Key = ANY(someArray)

I have minimal success with this, when I put in a WHERE clause such as

WHERE Bar.BarID = 3

My results are simply ‘Winner’m but I also want to get ‘Participant’. If I set up my WHERE to include nulls:

WHERE 
    Bar.BarID = 3
  OR
    Foo.someArray = null

I still do not get my null values out.

No where clause returns all combinations of values, including the nulls.

I’m not really sure how to get this behaviour I want. I can get all, or I can get just one set of values, but not both.

To give examples, if I were to do the join for BarID = 1, I should get “Winner”, “Grand prize” and “participant”. If I were to select for BarID 5, I would only get Participant.

Answer 1

Try:

Foo.someArray IS NULL

The equality operator always yields NULL, not TRUE when applied to a NULL value. To test a column whether it holds the NULL value, you need the above expression, not Foo.someArray = NULL.

More about that in the manual.

You also have to put it in your JOIN condition, not in a WHERE clause, as @Michael commented. Like:

LEFT JOIN Bar ON Bar.Key = ANY(Foo.someArray) OR Foo.someArray IS NULL

If you put it in the WHERE clause, you might also find rows where Foo.someArray IS NOT NULL, but has different prizes. With a LEFT JOIN, if (and only if) no row is found in Foo for a given Bar, NULL values will be appended for all columns of Foo, including someArray. This would fool a check for NULL in a WHERE condition.