T-SQL

You Shoulda Brought A Bigger Int Sometimes you run a query, and everything goes fine. For a while. For example, if I run this query in the 2010 copy of Stack Overflow, it finishes pretty quickly, and without error. Transact-SQL SELECT u.Id, u.DisplayName, SUM(p.Score) AS SumPostScore, AVG(c.Score) AS SumCommentScore FROM dbo.Users AS u JOIN dbo.Posts…

What’s New in SQL Server 2019: Faster Table Variables (And New Parameter Sniffing Issues)

Last Updated May 26, 2020

Parameter Sniffing, SQL Server 2019, T-SQL

For over a decade, SQL Server’s handling of table variables has been legendarily bad. I’ve long used this Stack Overflow query from Sam Saffron to illustrate terrible cardinality estimation: Transact-SQL declare @VoteStats table (PostId int, up int, down int) insert @VoteStats select PostId, up = sum(case when VoteTypeId = 2 then 1 else 0 end), down…

How to Check for Non-Existence of Rows

Last Updated July 20, 2018

Execution Plans, T-SQL

You’re writing a query, and you wanna check to see if rows exist in a table. I’m using the free Stack Overflow database, and I wanna find all of the users who have not left a comment. The tables involved are: In dbo.Users, the Id field uniquely identifies a user. In dbo.Comments, there’s a UserId field…

Stupid T-SQL Tricks

Last Updated May 30, 2018

Bad Idea Jeans, T-SQL

Presented without comment: Transact-SQL CREATE TABLE dbo.[FROM] ([SELECT] INT, [WHERE] INT, [LIKE] INT); GO SELECT [SELECT] FROM [FROM] WHERE [WHERE] LIKE [LIKE]; GO 1234 CREATE TABLE dbo.[FROM] ([SELECT] INT, [WHERE] INT, [LIKE] INT);GOSELECT [SELECT] FROM [FROM] WHERE [WHERE] LIKE [LIKE];GO Next up, can you break up a query with spaces? Yep: Transact-SQL SELECT * FROM…

How to Reduce the CPU Overhead of Dynamic SQL

Last Updated May 23, 2018

Tara Kizer

Development, T-SQL

Dynamic SQL is a good choice for catch-all type queries, but you have to be careful how you execute the dynamic string. For frequently executed queries, “EXEC sp_executesql @sql”is a good choice but at the risk of encountering parameter sniffing issues. For queries that aren’t executed very often, “EXEC (@sql)” can be used, but you…

Quick Tips For Debugging Large Stored Procedures

Last Updated May 16, 2018

Hell Here! Get Your Fresh Hell, Here! Let’s face it — as far as developer tools go, SSMS is pretty bad. Intellisense? The jokes write themselves. Don’t get me wrong, SSMS is a good management tool, but it’s not a good development tool (and what kind of maniac sticks debug — F6 — next to execute —…

How to Get a Random Row from a Large Table

Last Updated March 7, 2018

Method 1, Bad: ORDER BY NEWID() Easy to write, but it performs like hot, hot garbage because it scans the entire clustered index, calculating NEWID() on every row: The plan with the scan That took 6 seconds on my machine, going parallel across multiple threads, using tens of seconds of CPU for all that computing…

Fifteen Things I Hate About ISNUMERIC

Last Updated February 26, 2018

Yello! Transact-SQL SELECT ISNUMERIC('$') AS [What] UNION ALL SELECT ISNUMERIC('£') AS [What] UNION ALL SELECT ISNUMERIC(',') AS [What] UNION ALL SELECT ISNUMERIC('.') AS [What] UNION ALL SELECT ISNUMERIC('0e+99') AS [What] UNION ALL SELECT ISNUMERIC('2e2') AS [What] UNION ALL SELECT ISNUMERIC('12D4') AS [What] UNION ALL SELECT ISNUMERIC(',1,1,1,1,1,1,1') AS [What] UNION ALL SELECT ISNUMERIC('-') AS [What] UNION…

Using WITH (NOEXPAND) to Get Parallelism with Scalar UDFs in Indexed Views

Last Updated February 12, 2018

Scalar functions are the butt of everybody’s jokes: their costs are wrong, their STATS IO results are wrong, they stop parallelism when they’re in check constraints, their stats are wrong in 2017 CU3, they stop parallelism in index rebuilds and CHECKDB, I could go on and on. Recently, we ran across yet another scenario where…

Creating Insert Triggers to Silently Ignore Data You Don’t Want

Last Updated February 1, 2018

Bad Idea Jeans, T-SQL

Say you’ve got an application that insists on inserting data into the database, and…you don’t want the data. You want the application to THINK it inserted the data – you don’t want to roll it back or return an error to the end user. You just don’t want the data, and you don’t want the…

Using LIKE on Integers Gets You Implicit Conversion

Last Updated February 1, 2018

Using the Stack Overflow public export, take these two queries looking for a particular user by Id (the clustering key): Transact-SQL SELECT * FROM dbo.Users WHERE Id = 26837; SELECT * FROM dbo.Users WHERE Id LIKE 26837; 12 SELECT * FROM dbo.Users WHERE Id = 26837;SELECT * FROM dbo.Users WHERE Id LIKE 26837; The first…

Two Code Patterns That Don’t Virtualize Well

Last Updated January 19, 2018

Virtualization used to be a really Big Deal™ for database admins: we had to do a lot of careful planning to get a virtualization project done right. These days, virtualization is more and more of a no-brainer: most apps make the transition just fine. Every now and then, though, an exception pops up – usually…

#TSQL2sday: Brad McGehee Made a Difference in My Career.

Last Updated November 8, 2017

For this month’s T-SQL Tuesday, Ewald asked who’s made a difference in our careers. When I first got started out in SQL Server, all I had was books and Books Online. Back then, neither of them were particularly well-indexed, nor were they up to date. Then I found a web site that turned things around.…

SQL Server Workarounds

Last Updated October 12, 2017

Development, SQL Server, T-SQL

360 Questions We’ve been asking you folks a lot about what you’d add to SQL Server, and we’ve gotten some great answers. There are even some that have been answered in the last couple versions of SQL Server. For instance, 2016 brought us STRING_SPLIT! SQL Server 2017 has a whole bunch of neat stuff: CONCAT_WS…

Bad Idea Jeans: Multiple Index Hints

Last Updated May 23, 2017

Bad Idea Jeans, T-SQL

I hate that you can do this Here’s the thing: I’m mostly writing this because I didn’t know you could do it. But it’s cool, because it’ll reinforce some other concepts, and I’ll show you why you shouldn’t do it. I’m talking, of course, about index hints. To be more specific, hinting multiple indexes on…

Don’t Use Scalar Functions in Views.

Last Updated May 1, 2021

The short story: if your view has a scalar user-defined function it it, any query that calls the view will go single-threaded, even if the query doesn’t reference the scalar function. Now for the long story. Quite often people will inherit and rely on views written back in the dark ages, before people were aware…

Announcing Erik’s First Video Class: T-SQL Level Up, Level 2

Last Updated April 23, 2017

Company News, T-SQL

You’re confident writing queries to get the data you want, and you’ve been doing it for a couple of years. You’ve taken our free T-SQL Level Up class, mastered tally tables and windowing functions, but what else is there? In this class with T-SQL maestro Erik Darling, you’ll learn tricks to take your queries to…

Replacing ISNULL In A WHERE Clause

Last Updated April 7, 2017

Execution Plans, T-SQL

I like blogging Really, I do. But sometimes I like a change of pace. Instead of blogging about this question, I posted it over on dba.stackexchange.com. Also, I thought it was an interesting question, and apparently many other people agreed. Not just because I wanted some more points, but because blog comments are notoriously sucky…

The Surprising Behavior of Trailing Spaces

Last Updated April 11, 2017