多个关系和MySQL查询

I'm currently working on a databank project with mysql and php where one particular query is giving me an enormous headache.

In this project we have to search subcontractors for skills like java, c++ etc and evaluate if their skill rating is equal or grater to the rating value asked for.

We have a database scheme which looks something like that:

table subs:

• sub_id
• sub_region_id

table regions:

• region_id
• region_name

table skills:

• skill_id
• skill_name

table skillratings:

• skillrating_id
• skillrating_value

table subtoskill:

• subtoskill_id
• subtoskill_sub_id
• subtoskill_skill_id
• subtoskill_raitng_id

The subtoskill Table holds the N*N relations of subcontractors to skills, so an entry with "1, 2, 3, 4" would state that the subcontractor with id 2 has the skill with id 3 and the rating with id 4.

Now I want to query multiple skills with an AND or OR connection, for example "which subcontractors have skill java with value>=2 AND c++ with value>=4".

The OR query works perfectly(code below) but the AND query gives 0 results, except if the same skill with different values is picked.

/*
 * selectSubsBySkill
 * search database for subcontractors with given skills and ratings
 * parameter:
 * -$skill_ids array of ids(int) of skills, same size as $ratings
 * -$ratings array of rating values(int) for skills, same size as $skill_ids
 * -$and boolean true if and connection, else or connection of skills
 * return:
 * array with matching subcontractors information
 */
public function selectSubsBySkill($skill_ids, $ratings, $and){
    if(empty($skill_ids)){
        return $this->selectSubs();
    }
    $sel = 'SELECT * FROM '.SUBS.','.REGIONS.','.SKILLS.','.SUBTOSKILL.','.SKILLRATINGS.' WHERE '.
        SUB_REGION_ID.'='.REGION_ID.' AND '.SUBTOSKILL_SUB_ID.'='.SUB_ID.' AND '.
    SUBTOSKILL_SKILL_ID.'='.SKILL_ID.' AND '.SKILLRATING_ID.'='.SUBTOSKILL_RATING_ID.' AND (';

    $i=0;
    foreach($skill_ids as $skill_id){
        $sel .= '('.SKILLRATING_VALUE.'>='.$ratings[$i].' AND '.SUBTOSKILL_SKILL_ID.'='.$skill_id.')';
        if($i<sizeof($skill_ids)-1){
            if($and)$sel .= ' AND ';
            else $sel .= ' OR ';
        }
        $i++;
    }       
    $sel .= ') GROUP BY '.SUB_ID;
    return $this->query($sel);
}

I've tried several other examples including queries with UNIONs or JOINs but still nothing has worked.

I'd be glad if someone here got a hint to solve this problem.