If D is 0, the result has no decimal point or fractional part.
D can be negative to cause D digits left of the decimal point of the value X to become zero.
mysql>
mysql> SELECT TRUNCATE(1.223,1);
+-------------------+
| TRUNCATE(1.223,1) |
+-------------------+
| 1.2 |
+-------------------+
1 row in set (0.00 sec)
mysql>
mysql> SELECT TRUNCATE(1.999,1);
+-------------------+
| TRUNCATE(1.999,1) |
+-------------------+
| 1.9 |
+-------------------+
1 row in set (0.00 sec)
mysql>