Deleting Duplicate Rows in a SQL Server Table Using a CTE

CTE example

Finding duplicate rows in a table is quite simple. Deleting only the duplicates – as opposed to deleting all rows that have duplicates – is a bit more complicated.

I have a table variable called “@Readings” with the following schema:

	TagName VARCHAR(100),
	TagValue NUMERIC(10,3)

First, I load the table with data from another database on a linked server called “OtherDB”:

INSERT INTO @Readings(TagName,ScanTime,TagValue)
SELECT DISTINCT tag,[time],value
	OPENQUERY(OtherDB,'select tag, time, value from archive.comp where (tag LIKE ''%test%'') AND time = DATE(''*'') ')

Then, I load that same data with some updates, both to the ScanTime column, and then to the TagValue column – always holding the TagName column constant.

SET @DateTimeStamp = GETDATE()

INSERT INTO @Readings(TagName,ScanTime,TagValue)
SELECT TagName,@DateTimeStamp,TagValue FROM @Readings

INSERT INTO @Readings(TagName,ScanTime,TagValue)
SELECT TagName,ScanTime,12345 FROM @Readings

To see what rows duplicate the TagName column alone, this query will suffice:

SELECT TagName FROM @PI_Readings GROUP BY TagName HAVING ( COUNT(*) > 1 )

By adding column names, you can look where the duplication is the TagName and the ScanTime:

SELECT TagName, ScanTime FROM @PI_Readings GROUP BY TagName, ScanTime HAVING ( COUNT(*) > 1 )

(If I were to add also the TagValue column, I would get zero rows returned, since there are currently no duplicates where all three columns are identical.)

So, what if you wanted to delete only the duplicates where TagName and ScanTime matched, irrespective of the TagValue?

The easiest way I have found is to use what is called a Common Table Expression, or CTE.

To use a CTE to delete these duplicates let’s first construct it to only show the duplicates:

	SELECT TagName, ScanTime, TagValue,
	FROM @Readings

To now delete the offending rows, change the last line from a SELECT statement to a DELETE statement:

CTE example

When creating the CTE statement, make sure there is a semicolon terminating the previous statement. This is not usually required in SQL Server, but it is in this case.

Another good example of this is on Stack Overflow.

While I do like SQL Fiddle, it seems that the DELETE function does not work on CTEs there, though SELECT statements do.

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.