Add Shorthand Absolute Value Operator as a postgres til

postgres/shorthand-absolute-value-operator.md

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+# Shorthand Absolute Value Operator
+
+Postgres offers many [math
+functions](https://www.postgresql.org/docs/8.0/functions-math.html) including
+`abs` for computing the absolute value of a number.
+
+```sql
+> select abs(-1);
+ abs
+-----
+   1
+(1 row)
+```
+
+There is also an absolute value _operator_ -- the `@` symbol. This can be used
+to do the same thing.
+
+```sql
+> select @ -1;
+ ?column?
+----------
+        1
+(1 row)
+```