Add The nullif Function as a Postgres til

postgres/the-nullif-function.md

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+# The nullif Function
+
+PostgreSQL, in addition to generalized case statements, includes the
+[`nullif`](https://www.postgresql.org/docs/current/functions-conditional.html)
+function. The docs describe it as a way "to perform the inversation operation
+of a `coalesce`".
+
+Rather than resolving to some fallback value if the primary value is `null`
+(like `coalesce` does), it will resolve to `null` if the given values are the
+same.
+
+```sql
+> select nullif(0, 0);
+ nullif
+--------
+      ø
+(1 row)
+```
+
+If the values are not equal, then the first value is the result of the
+function.
+
+```sql
+> select nullif(1, 0);
+ nullif
+--------
+      1
+(1 row)
+```
+
+One way this can be used is in conjunction with the `coalesce` function. For
+instance, if I have a table of values that are either 0 or a positive number, I
+can coerce all the zeros to be `1` like so.
+
+```sql
+> select coalesce(nullif(0, 0), 1);
+ coalesce
+----------
+        1
+(1 row)
+```
+
+h/t [Ian Jones](https://twitter.com/_jonesian)