Syntax:
INTERVAL(N,N1,N2,N3,...)

Returns 0 if N < N1, 1 if N < N2 and so on or -1 if N is NULL. All
arguments are treated as integers. It is required that N1 < N2 < N3 <
... < Nn for this function to work correctly. This is because a binary
search is used (very fast).

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

