Add Different Ways To Define An Interval as a PostgreSQL TIL

postgres/different-ways-to-define-an-interval.md

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+# Different Ways To Define An Interval
+
+There are several different ways in PostgreSQL to define an `interval` data
+type. An `interval` is useful because it can represent a discrete chunk of
+time. This is handy for doing date math.
+
+Here are four different ways to define an `interval`:
+
+1. Use the `interval` keyword with a string
+
+```sql
+> select interval '3 days';
+ interval
+----------
+ 3 days
+(1 row)
+```
+
+2. Cast a string to the `interval` type
+
+```sql
+> select '3 days'::interval;
+ interval
+----------
+ 3 days
+(1 row)
+```
+
+3. The `@` operator is a finicky syntax for declaring an interval
+
+```sql
+> select @ 3 days;
+ days
+------
+    3
+(1 row)
+```
+
+4. The [`make_interval`
+   function](https://www.postgresql.org/docs/current/functions-datetime.html)
+   can take various forms of arguments to construct an interval
+
+```sql
+> select make_interval(days => 3);
+ make_interval
+---------------
+ 3 days
+(1 row)
+```
+
+[source](https://www.postgresql.org/docs/current/datatype-datetime.html#DATATYPE-INTERVAL-INPUT)