ANOTHER SQL Post this week? I got nerd-sniped:

Oooooh, challenge accepted!

So, let’s assume we have a table `T`

with columns `(A, B, C)`

like this:

WITH t(a, b, c) AS ( SELECT 'a', 'b', null FROM dual UNION ALL SELECT 'a', null, 'c' FROM dual UNION ALL SELECT 'a', 'b', 'c' FROM dual ) SELECT * FROM t

As expected, this yields:

A B C ----------------- a b a c a b c

Truly exciting.

Now we want to find all those rows that “match” either `('a', 'b', NULL)`

or `('a', NULL, 'b')`

. Clearly, this should produce the first two rows, right?

A B C ----------------- a b a c

Yes. Now the canonical solution would be to tediously write out the entire predicate as such:

WITH t(a, b, c) AS (...) SELECT * FROM t WHERE (a = 'a' AND b = 'b' AND c IS NULL) OR (a = 'a' AND b IS NULL AND c = 'c')

That’s really boring. Sure, we could have factored out the first, common predicate:

WITH t(a, b, c) AS (...) SELECT * FROM t WHERE a = 'a' AND ( (b = 'b' AND c IS NULL) OR (b IS NULL AND c = 'c') )

That’s certainly better from a performance perspective, but Rafael had a nifty idea. Let’s use row value expressions (tuples) in our predicates:

WITH t(a, b, c) AS (...) SELECT * FROM t WHERE (a, b, c) IN (('a', 'b', NULL), ('a', NULL, 'c'))

Unfortunately this doesn’t yield any results, because nothing is equal to `NULL`

in SQL (not even `NULL`

itself). The above query is the same as this one:

WITH t(a, b, c) AS (...) SELECT * FROM t WHERE (a = 'a' AND b = 'b' AND c = NULL /* oops */) OR (a = 'a' AND b = NULL /* oops */ AND c = 'c')

D’oh.

### Solutions

**The lame one**

The canonical solution then would be a really lame (but perfectly valid) one. Encode `NULL`

to be some “impossible” string value. Rafael suggested `yolo`

. Fair enough.

This works:

WITH t(a, b, c) AS (...) SELECT * FROM t WHERE (a, NVL(b, 'yolo'), NVL(c, 'yolo')) IN (('a', 'b', 'yolo'), ('a', 'yolo', 'c'))

All we have to do now is to always remember the term `yolo`

which means “`NULL`

, but not `NULL`

thank you SQL”

**The hipster one**

But wait! SQL is painstakingly inconsistent when it comes to `NULL`

. See, `NULL`

really means `UNKNOWN`

in three-valued logic, and this means, we never *know* if SQL abides to its own rules.

Come in `INTERSECT`

. Like `UNION`

or `EXCEPT`

(`MINUS`

) in Oracle, as well as `SELECT DISTINCT`

, these set operations handle two `NULL`

values as `NOT DISTINCT`

. Yes, they’re not equal but also not distinct. Whatever. Just remember: That’s how it is 🙂

So, we can write this hipster solution to Rafael’s problem:

WITH t(a, b, c) AS (...) SELECT * FROM t WHERE EXISTS ( SELECT a, b, c FROM dual INTERSECT ( SELECT 'a', 'b', null FROM dual UNION ALL SELECT 'a', null, 'c' FROM dual ) )

We create an intersection of the tuple `(a, b, c)`

, the left side of Rafael’s `IN`

predicate, and the desired values on the right side of the `IN`

predicate, and we’re done.

Clearly less tedious than writing the original predicates, right? (We won’t look into performance this time)

Cheers, and a happy weekend.