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 any argument is NULL, the result is NULL. No comparison is needed.

o If all arguments are integer-valued, they are compared as integers.

o If at least one argument is double precision, they are compared as
  double-precision values. Otherwise, if at least one argument is a
  DECIMAL value, they are compared as DECIMAL values.

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.

The return type of LEAST() is the aggregated type of the comparison
argument types.

URL: http://dev.mysql.com/doc/refman/5.5/en/comparison-operators.html

