Insert data only if record does not exist

Question

I want to insert data in a table only if the record with a given CatalogID does not exist.

 $userId = $_POST[\'userId\'];
 $catalogID = $_POST[\         


        

Answer 1

$query="INSERT INTO LibraryMaster (UserID,CatalogID,ContentAddedDateTime)
SELECT * FROM (SELECT '".$userId."', '".$catalogID."', '".$content_AddedTime."') AS tmp
WHERE NOT EXISTS (
  SELECT CatalogID FROM LibraryMaster WHERE CatalogID = '".$catalogID."t'
 ) LIMIT 1";