ABS(X)

ABS()函数返回X的绝对值，请考虑以下的例子：

mysql> SELECT ABS(2);
+---------------------------------------------------------+
| ABS(2)                                                  |
+---------------------------------------------------------+
| 2                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)

mysql> SELECT ABS(-2);
+---------------------------------------------------------+
| ABS(2)                                                  |
+---------------------------------------------------------+
| 2                                                       |
+---------------------------------------------------------+
1 row in set (0.00 sec)