how to add custom fields in post table

I added custom fields in post table manually and inserting the values using my own query.Wat i observed is,on admin side,when admin edit a post,it makes a new entry in post table and apply new changes in tht row.Problem i m facing it,it doesn’t sustain my custom fields values which i added manually and insert Null in those fields.Whats an easy way of
resolving this issue ?
Should i search for the code on admin folder where it updates post value in new row and add my custom fields values or is there any plugin available which will insert those custom fields from admin side ?