Monthly Archives: April 2019

A do not use sign signified by a cartoon man showing his hand within a red circle with a red line dividing it in two

How to NOT write a T-SQL query

Database Administrator, dba, sql, SQL Server, T-SQL, , , , , , , , , , , ,

So the vendors for the call system software ran the code below on the live system on a Friday night. Thanks guys. I’ve changed the name of the tables in the code for security reasons if anyone would like to use this it as an example of how not to write T-SQL code!

The vendors claim the code came from the company that developed the call system and if this is the case I think someone in head office is trying to get fired or get someone fired.

Scroll down for a review of why the code is such a mess.

/*
TableOne a 312 million row table
TableTwo a 55 million row table 
TableThree a 22 million row table
*/

DELETE
FROM TableOne
WHERE SecurityPolicyId = @p_secPolId
	AND RecordingId NOT IN (
		SELECT c.RECORDINGID
		FROM TableThree c
		WHERE (
				EXISTS (
					SELECT *
					FROM TableTwo b0
					WHERE b0.RecordingID = c.RECORDINGID
						AND b0.Workgroup = N'CS EMAIL - [Cancellations]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b1
					WHERE b1.RecordingID = c.RECORDINGID
						AND b1.Workgroup = N'CS EMAIL - [Claims]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b2
					WHERE b2.RecordingID = c.RECORDINGID
						AND b2.Workgroup = N'CS EMAIL - [Contact Request]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b3
					WHERE b3.RecordingID = c.RECORDINGID
						AND b3.Workgroup = N'CS EMAIL - [DD Enquiries]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b4
					WHERE b4.RecordingID = c.RECORDINGID
						AND b4.Workgroup = N'CS EMAIL - [Feedback]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b5
					WHERE b5.RecordingID = c.RECORDINGID
						AND b5.Workgroup = N'CS EMAIL - [MTA]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b6
					WHERE b6.RecordingID = c.RECORDINGID
						AND b6.Workgroup = N'CS EMAIL - [NB Cover Query]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b7
					WHERE b7.RecordingID = c.RECORDINGID
						AND b7.Workgroup = N'CS EMAIL - [O/S Documents]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b8
					WHERE b8.RecordingID = c.RECORDINGID
						AND b8.Workgroup = N'CS EMAIL - [Otherl]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b9
					WHERE b9.RecordingID = c.RECORDINGID
						AND b9.Workgroup = N'CS EMAIL - [SME]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b10
					WHERE b10.RecordingID = c.RECORDINGID
						AND b10.Workgroup = N'CS WebChat'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b11
					WHERE b11.RecordingID = c.RECORDINGID
						AND b11.Workgroup = N'CUSTOMER SERVICE - [Cancellations]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b12
					WHERE b12.RecordingID = c.RECORDINGID
						AND b12.Workgroup = N'CUSTOMER SERVICE - [CBL enquiry]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b13
					WHERE b13.RecordingID = c.RECORDINGID
						AND b13.Workgroup = N'CUSTOMER SERVICE - [Claims]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b14
					WHERE b14.RecordingID = c.RECORDINGID
						AND b14.Workgroup = N'CUSTOMER SERVICE - [Commercial]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b15
					WHERE b15.RecordingID = c.RECORDINGID
						AND b15.Workgroup = N'CUSTOMER SERVICE - [DD Payment]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b16
					WHERE b16.RecordingID = c.RECORDINGID
						AND b16.Workgroup = N'CUSTOMER SERVICE - [Diary Team]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b17
					WHERE b17.RecordingID = c.RECORDINGID
						AND b17.Workgroup = N'CUSTOMER SERVICE - [Doc Request]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b18
					WHERE b18.RecordingID = c.RECORDINGID
						AND b18.Workgroup = N'CUSTOMER SERVICE - [DocChase CL]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b19
					WHERE b19.RecordingID = c.RECORDINGID
						AND b19.Workgroup = N'CUSTOMER SERVICE - [DocChase FN]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b20
					WHERE b20.RecordingID = c.RECORDINGID
						AND b20.Workgroup = N'CUSTOMER SERVICE - [DocChase IN]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b21
					WHERE b21.RecordingID = c.RECORDINGID
						AND b21.Workgroup = N'CUSTOMER SERVICE - [Life]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b22
					WHERE b22.RecordingID = c.RECORDINGID
						AND b22.Workgroup = N'CUSTOMER SERVICE - [MTA]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b23
					WHERE b23.RecordingID = c.RECORDINGID
						AND b23.Workgroup = N'CUSTOMER SERVICE - [NB Cover Query]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b24
					WHERE b24.RecordingID = c.RECORDINGID
						AND b24.Workgroup = N'CUSTOMER SERVICE - [Other Query]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b25
					WHERE b25.RecordingID = c.RECORDINGID
						AND b25.Workgroup = N'Customer Service Admin'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b26
					WHERE b26.RecordingID = c.RECORDINGID
						AND b26.Workgroup = N'OUTBOUND - [Welcome Calls]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b27
					WHERE b27.RecordingID = c.RECORDINGID
						AND b27.Workgroup = N'OUTBOUND CS - [Cancelations]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b28
					WHERE b28.RecordingID = c.RECORDINGID
						AND b28.Workgroup = N'OUTBOUND CS - [DocChase]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b29
					WHERE b29.RecordingID = c.RECORDINGID
						AND b29.Workgroup = N'OUTBOUND CS - [Final Notice]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b30
					WHERE b30.RecordingID = c.RECORDINGID
						AND b30.Workgroup = N'OUTBOUND CS - [Initial Chase]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b31
					WHERE b31.RecordingID = c.RECORDINGID
						AND b31.Workgroup = N'OVERFLOW - [Claims]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b32
					WHERE b32.RecordingID = c.RECORDINGID
						AND b32.Workgroup = N'OVERFLOW - [Customer Service - MTA]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b33
					WHERE b33.RecordingID = c.RECORDINGID
						AND b33.Workgroup = N'RENEWALS - [Personal Enquiries]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b34
					WHERE b34.RecordingID = c.RECORDINGID
						AND b34.Workgroup = N'RENEWALS - [Personal Outbound]'
					)
				OR EXISTS (
					SELECT *
					FROM TableTwo b35
					WHERE b35.RecordingID = c.RECORDINGID
						AND b35.Workgroup = N'RENEWALS - [Personal Payments]'
					)
				)
			AND c.RecordingDate >= @p_dateAfter
			AND c.RecordingDate < @p_dateBefore
			OR c.RecordingDate < @p_dateAfter
			OR c.RecordingDate >= @p_dateBefore
		)

 

Here are some of the issues with the above in no particular order.

The command is running against a 312 million row table, a 55 million row table and a 22 million row table. Although is was only planned to run once IT STILL NEEDS TO BE EFFICIENT!!!!!

It uses Select Star or (Select *) which raises the probability that SQL Server will query the whole table rather than accessing the data through indexes. Only an Id is needed to begin with.

There are subqueries used when a join would be much faster.

The subqueries are completely unnecessary when WHERE conditions could have been listed in an IN statement.

Correlated subquery!!! Literally designed to bring a server to its knees! Evaluated once for each row processed. See more on correlated sub queries here https://en.wikipedia.org/wiki/Correlated_subquery

There are EXISTS Statements used due to the poor design of the command.

A ridiculously giant case statement used! Typically the comparison data should be written to a temp table and assessed as a join when there are an excessive number of case statements.

Needless to say this query ballooned the tempdb and I’ll let you guess what happened next.

The lesson for today’s posting, trust no one!