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Category: T-SQL

Bit Twiddling in T-SQL

Published 2022-12-05 by Kevin Feasel

Louis Davidson explains how bit operations work in T-SQL:

I expect that 99% of the people reading this looks at this probably would expect there to be a status table that contained the values of status. Seeing that this is a base 2 number, you may be in that 1% that thinks this might be a bitmask. but unless you have and eidetic memory, you probably don’t know what all of the bits mean.

A bitmask is a type of denormalization of values where instead of having a set of columns that have on or off values (no Null values), you encode it like:

Bitmasks make me break out the angry nun ruler. You can almost guarantee you’re doing something wrong if you design a bitmask as a column in a table.

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GENERATE_SERIES and Data Types

Published 2022-11-23 by Kevin Feasel

Bill Fellows runs into an issue:

Perfect, now I have a row for each second from midnight to approximately 5.5 hours later. What if my duration need to vary because I’m going to compute these ranges for a number of different scenarios? I should make that 19565 into a variable and let’s overengineer this by making it a bigint.

Things don’t work out quite the way you might have expected there. Read on and see what Bill found and how you can circumvent the problem.

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AT TIME ZONE Performance and an Alternative

Published 2022-11-14 by Kevin Feasel

Joe Obbish looks at time zones:

Queries that use the AT TIME ZONE operator may perform worse than desired. For example, Jonathan Kehayias blogged about performance struggles with AT TIME ZONE at sqlskills.com. One key difference between Jonathan’s opinion and my own is that I enjoy writing these kinds of posts.

Read on for the details and check out some work Joe has done around time zone friendly functions as an alternative to AT TIME ZONE.

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Qualities of Production-Grade Code

Published 2022-11-09 by Kevin Feasel

Aaron Bertrand pulls out the list:

In a lot of programming languages, efficiency is almost always the guidepost. Sometimes, minimizing character count or line count is a “fool’s gold” measure of the quality of the code itself. Other times, unfortunately, engineers are judged not by quality at all, but rather the sheer number of lines of code produced – where more is, somehow, obviously better. Over my career, “how much code there is” has never been a very meaningful measure in any language.

But I’m here to talk about T-SQL, where certainly efficiency is a good thing to measure – though there are some caveats to that:

Read on for those caveats and what Aaron considers to be the hallmarks of high-quality code.

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Deleting Data from SQL Server

Published 2022-11-08 by Kevin Feasel

Greg Larsen fills us in on an important command:

Over time data in SQL Server tables needs to be modified. There are two major different aspects of modifying data: updating and deleting. In my last article “Updating SQL Server Data” I discussed using the UPDATE statement to change data in existing rows of a SQL Server table. In this article I will be demonstrating how to use the DELETE statement to remove rows from a SQL Server Table.

This stays pretty simple but provides an effective overview of how to keep those tables tidy.

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LATERAL and APPLY

Published 2022-11-04 by Kevin Feasel

Lukas Eder shows off one of my favorite operators:

The SQL:1999 standard specifies the <lateral derived table>, which is SQL’s way of allowing for a derived table (a subquery in the FROM clause) to access all the lexically preceding objects in the FROM clause. It’s a bit weird in terms of syntax, I personally think that Microsoft SQL Server has a much nicer solution for this concept via APPLY. Oracle supports both syntaxes (standard and T-SQL’s). Db2, Firebird, MySQL, PostgreSQL only have LATERAL.

Click through to see how the operator works.

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Non-Parallel Plans from Computed Columns with Scalar Functions

Published 2022-11-04 by Kevin Feasel

Etienne Lopes tells a tale:

I must say that per principle I’m not a big fan of neither computed columns nor scalar UDFs. I mean, I find them attractive in the way they (appear to) make “things simpler” also allowing code reuse, improving queries readability, etc. Yes but they also hide or mask the complexity behind their use, which can often be quite deceiving, making it much harder to troubleshoot and solve performance problems. Furthermore they have several limitations by design that can hurt performance and all this combined, can sometimes make a “simple” query take many minutes or hours to run, instead of just a few seconds! When you see this situation happen again and again while fine tuning databases, their use becomes much less appealing.

Having this said, sometimes they can be useful of course but it’s very important to choose carefully where, how and when to use computed columns and scalar UDFs, so that performance won’t get hurt and its benefits outweigh the drawbacks.

Click through for an example of where the combo really falls short. I do like computed columns, though never with user-defined functions.

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Using the T-SQL OUTPUT Clause

Published 2022-11-02 by Kevin Feasel

Chad Callihan doesn’t make two calls:

Are you familiar with the OUTPUT clause in SQL Server? It can be used with commands like INSERT, UPDATE, DELETE, and MERGE. It can also be used with setting variables in stored procedures. Using the tried and true StackOverflow2013, we’ll narrow it down today to focus on how INSERT/DELETE are typically used for logging table changes as well as an example of how to use OUTPUT with stored procedures.

For really busy transactional systems, this provides a nice boost over making an update and then selecting the new values.

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