GREATEST/LEAST(x,y,...) 
GREATEST(x,y,...) or LEAST(x,y) is a special function that calculates the maximum or minimum value of all arguments (expressions).
The functions can be applied to any data type. The data types of the individual expressions must be comparable.
x, y, ... |
Result of the Function |
|---|---|
At least one argument is the NULL value |
NULL value |
At least one argument is the special NULL value |
Special NULL value |
Example
SQL Tutorial, Information About Character Strings