Syntax:
LEAST(value1,value2,...)

With two or more arguments, returns the smallest (minimum-valued)
argument. The arguments are compared using the following rules:

o If the return value is used in an INTEGER context or all arguments
  are integer-valued, they are compared as integers.

o If the return value is used in a REAL context or all arguments are
  real-valued, they are compared as reals.

o If the arguments comprise a mix of numbers and strings, they are
  compared as numbers.

o If any argument is a nonbinary (character) string, the arguments are
  compared as nonbinary strings.

o In all other cases, the arguments are compared as binary strings.

Before MySQL 5.0.13, LEAST() returns NULL only if all arguments are
NULL. As of 5.0.13, it returns NULL if any argument is NULL.

URL: http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html

