SET @interval = CURRENT_DATE - INTERVAL 90 DAY;

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mysql> SET @interval = CURRENT_DATE - INTERVAL 90 DAY;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT @interval ;
+------------+
| @interval  |
+------------+
| 2016-12-30 |
+------------+
1 row in set (0.00 sec)

mysql>