Master keyword table connecting to multiple table

We have one keyword table and different tables connecting to the keyword table as the interface table (e.g **question_keyword**, **answer_keyword**). We would like to create one master keyword table so when the user searches for a word it can find the word in question table or answer table and retrieve information from that table. We thought of having one master_table(id, question_id, answer_id, keyword_id). Is this design correct? and if we have more tables (means more columns in master_table), how would be the performance (since it might make the master_table as sparse matrix)?