Assuming that your category column is like: "cats, dogs, elephants"; separated keywords. there are 3 solutions :
1) Ugliest solution, probably the worst :
WHERE category like CONCAT("%",@category1,"%")
OR category like CONCAT("%",@category2,"%")
But for this you should know how many categories you are gonna query for. This way probably wont work at all.
2) If its a myisam table you can use full-text search functions to match against category field which would be much more faster and tidier than 1st option.
3) The best way is to redesign table structure to have a relation table between categories and subject item (since it seems like a 1-N relationship)
If you can give some definitions (codes) and data samples that would help understanding.