Add The nullif Function as a Postgres til

Author: jbranchaud

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
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-_1134 TILs and counting..._
+_1135 TILs and counting..._
 
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@@ -622,6 +622,7 @@ _1134 TILs and counting..._
 - [Temporarily Disable Triggers](postgres/temporarily-disable-triggers.md)
 - [Temporary Tables](postgres/temporary-tables.md)
 - [Terminating A Connection](postgres/terminating-a-connection.md)
+- [The nullif Function](postgres/the-nullif-function.md)
 - [Timestamp Functions](postgres/timestamp-functions.md)
 - [Toggling The Pager In PSQL](postgres/toggling-the-pager-in-psql.md)
 - [Track psql History Separately Per Database](postgres/track-psql-history-separately-per-database.md)

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)