Java OOP:ตัดคลังสินค้าเเล้วติดลบ ต้องการให้ตรวจสอบก่อนตัดstock

import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class TestSQLcountdrug {
	public TestSQLcountdrug(String namedrug,String amount){
		Connection theCon = null;
		SQLConnecttion myCon;
		Statement stmt ;
		String SQL;
		ResultSet rs;
		
		
		try{
			myCon = new SQLConnecttion();
			theCon = myCon.getConnection();
			stmt = theCon.createStatement();
			SQL = "update drug set  amount = (select amount from drug where name = '"+namedrug+"')-'"+amount+"' where name = '"+namedrug+"'";
			rs = stmt.executeQuery(SQL);
			
		}catch(SQLException e){
			e.printStackTrace();
		}finally{
			try{
				if(theCon != null){
					theCon.close();
				}
			}catch(Exception e){
				e.printStackTrace();
			}
		}
	}
}