Using MAX() in a SQL Subquery

database diagram showing table relationships

One pro bono project I’m working on is improving a school website where parents can sign up for classes, view students’ grades, etc. One of the problems with the website was that the list of user accounts, which includes all parents and teachers, includes parents from previous years who no longer have students there.

The query for pulling this information was very simple:

SELECT * FROM UserAccounts ORDER BY lastName ASC

This query pulled all users and ordered them only by last name. The data was being dumped into an ASP.NET GridView with column sorting, and there were only a couple hundred people there, so it wasn’t completely unmanageable. However, dealing with all the parents who no longer have kids there did make visual searches more difficult. The request was to bring all current parents to the top of the list so that other admins don’t have to search through pages of people who no longer attend.

A quick fix for this would be to sort the list based on last academic year attended. We’ll do this by employing the MAX() function.

In addition to the UserAccounts table, there are other tables called “students” (which includes all current and past students) and “schedule” (which holds student schedules). In the schedule table, a column called “s_year” that holds the academic year (in the format of “2017-18”). We can join these tables based on user account IDs in the UserAccount table, the parentID column in the students table, and the student id (s_id) column in the schedule table.

One more thing: Since this list also includes people who have never had students there, the revised query will have to take that into account.

database diagram showing table relationships

The below query will not only solve this problem, but will also sort by first and last name. Users with no academic year (a NULL value) will be at the bottom of the list.

SELECT DISTINCT ua.*, sc.s_year FROM UserAccounts ua 
LEFT OUTER JOIN students s ON s.parentID = ua.ID 
LEFT OUTER JOIN schedule sc ON sc.s_id = s.ID 
WHERE sc.s_year IS NULL 
OR sc.s_year = (
   SELECT MAX(sc2.s_year) FROM schedule sc2 WHERE sc2.s_id = s.ID
) 
ORDER BY sc.s_year DESC, ua.lastName, ua.firstName

The only remaining thing to do is to add the year column to the GridView and make sure that sorting is enabled.

EF Database First with ASP.NET MVC 5 and Oracle Database 12c

NuGet Package Manager

There is a great article called “EF Database First with ASP.NET MVC” by Tom FitzMacken that is part of Microsoft’s ASP.NET MVC tutorial.

I wanted to create a website using the steps outlined here, except instead of using a SQL Server database (as was presented in the article), I wanted to use an Oracle 12c database. Unfortunately, Oracle was not an option in the list of Data Sources as in the image below:

no Oracle option available
Oracle DB was not an option.

I had the basic Oracle setup on my development server, however, I found that I did not have Oracle Developer Tools for Visual Studio. I installed the 32-bit version and rebooted the server; I have not tested the 64-bit version for this particular project. Also, I changed the .NET Framework from 4.5 to 4.6. (From what I have read, 4.52 is the minimum that will work with the EF 6 / Oracle setup.) Lastly, I installed several NuGet packages:

  1. Official Oracle ODP.NET, Managed Driver
  2. Official Oracle ODP.NET, Managed Entity Framework Driver
  3. Oracle Data Provider for .NET (ODP.NET) Managed Driver
  4. Oracle Rdb Entity Framework Provider
NuGet Package Manager
NuGet Package Manager

I closed and reopened Visual Studio 2013, and reopened the solution I had created. Now, when adding a Model as in step 2 of the article, I had Oracle options in the Data Source list!

an Oracle option is available
Oracle is now an option!

After selecting the Oracle option, you can complete setting up the connection as below. TNS is available as an option; if you use this, the connection string to Oracle must bu set up in your TNSnames file.

Connection Properties dialog box
Select your Oracle schema.

After this is done, you can continue with the linked tutorial above!