Add Is It Null Or Not Null as a postgres til.

postgres/is-it-null-or-not-null.md

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+# Is It Null Or Not Null?
+
+In PostgreSQL, the standard way to check if something is `NULL` is like so:
+
+```sql
+select * as wild_pokemons from pokemons where trainer_id is null;
+```
+
+To check if something is not null, you just add `not`:
+
+```sql
+select * as captured_pokemons from pokemons where trainer_id is not null;
+```
+
+PostgreSQL also comes with `ISNULL` and `NOTNULL` which are non-standard
+ways of doing the same as above:
+
+
+```sql
+select * as wild_pokemons from pokemons where trainer_id isnull;
+```
+
+```sql
+select * as captured_pokemons from pokemons where trainer_id notnull;
+```