A General Database Question. Please Help Urgent!

guys i have a programming question i have table called prerequsiteswhich is like thisprerequisite component                       componenta                                                         bso inside my before insert trigger i have to check if the prerequisite is a valid entry like for instance an entry like b is a prerequisite component for component a which is not a valid entry since i already have an entry in the prerequisite table which defines that comonent a is a prerequisite of b. however this is very basic so i can test it easy some thing like this in my before insert prerequisite component                       componenta                                                         bb                                                         a (  Not a valid entry)select count(*)  from prerequisite where component = value of prerequisite component  and prerequisite component  = value of component the above select statement will actually tell me if entry is allowed for insert and does not override other prerequsitesbut it can get very complex depending on how the user chooses so if someone can tell me a nice algorithm or good logic to approach this problem it would be great like for instanceprerequisite component                       componenta                                                         bb                                                         cc                                                         a ( Not a valid entry. so how do i ensure that this entry is not allowed considering that complexity for prerequsites can increase quite greatly)please can someone tell me how to approach this problem.