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 any argument is a case-sensitive string, the arguments are
  compared as case-sensitive strings.

o In all other cases, the arguments are compared as case-insensitive
  strings.

LEAST() returns NULL if any argument is NULL.

URL: http://dev.mysql.com/doc/refman/5.1/en/comparison-operators.html

