Make a receive data connection for your sql table, Take one more textBox field and name it as result field and by default make its value false.
Then write a rule on you previous field. In rule conditition select a specific column from your sql table and then check if your sql table column contains your field value, If yes then set your result field values as true.
So now by your result field value you can know whether your entered text is already in the sql table or not.