Add Checking Inequality as a postgres til

postgres/checking-inequality.md

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+# Checking Inequality
+
+In most languages there is a `!=` operator for checking inequality of two
+things.
+
+Postgres also supports the synonymous `<>` operator for checking inequality.
+
+```sql
+> select 1 <> 1;
+ ?column?
+----------
+ f
+
+> select true <> false;
+ ?column?
+----------
+ t
+
+> select 'taco' <> 'burrito';
+ ?column?
+----------
+ t
+```
+
+h/t Brian Dunn
+
+[source](https://www.postgresql.org/docs/9.5/static/functions-comparison.html)