Software Engineering for Internet Applications - Student Community

create table km_object_views (object_view_id integer primary key,-- which useruser_idnot null references users,-- two columns to specify which objectobject_id integer not null,table_name varchar(21) not null,view_time timestamp(0) not null,reuse_pchar(1) default 'f'check(reuse_p in ('t','f')));Modify object-view-one so that it will insert a row into thekm_object_views table if and only if there isn't already a log rowfor this user/object pair within 24 hours. You can do this with thefollowing procedure:2801. open a transaction2. lock the table3. count the number of matching rows within the last 24 hours4. compare the result to 0 and insert if necessary5. close the transactionThis appears to be an awfully big hammer for a seemingly simpleproblem. Is it possible to do this in one statement?Let's start with Oracle. Here's an example of an INSERT statementthat only has an effect if there isn't already a row in the table:insert into km_object_views (object_view_id,user_id, object_id, table_name, view_time)select km_object_view_id.nextval, 227, 891,'algorithm', current_timestamp(0)from dualwhere 0 = (select count(*)from km_object_viewswhere user_id = 227and object_id = 891and view_time > current_timestamp -interval '1' day);The structure of this statement is "insert into KM_OBJECT_VIEWSthe result of querying the 1-row system table DUAL". We're notpulling any data from the DUAL table, only including constants in theSELECT list. Nor is the WHERE clause restricting results based oninformation in the DUAL table; it is querying KM_OBJECT_VIEWS.{object objResult = null;string strResult = null;string strEmployeeID = "PMA42628M";//Initialize the database connection,// command and parameter objects.SqlConnection conn = new SqlConnection(ConfigurationSettings.AppSettings["connStr"]);SqlCommand cmd = new SqlCommand("select fname from employee where emp_id =@emp_id");SqlParameter param = newSqlParameter("@emp_id",strEmployeeID);//Associate the connection with the command.cmd.Connection = conn;//Bind the parameter value to the command.cmd.Parameters.Add(param);//Connect to the database and// run the command.try{conn.Open();objResult = cmd.ExecuteScalar();}catch (Exception e){Console.WriteLine("Database error: {0}",e.ToString());}finally{//Clean up.if(!conn.State.Equals(ConnectionState.Closed)){conn.Close();}}//Convert the query result to a string.if (objResult == null)69