DRDS 支持如下比较函数：
| 函数名 | 描述 |
| COALESCE() | Return the first non-NULL argument |
| GREATEST() | Return the largest argument |
| IN() | Check whether a value is within a set of values |
| INTERVAL() | Return the index of the argument that is less than the first argument |
| ISNULL() | Test whether the argument is NULL |
| LEAST() | Return the smallest argument |
| NOT IN() | Check whether a value is not within a set of values |
| STRCMP() | Compare two strings |
