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Category: T-SQL

Using a Trigger to Auto-Refresh View Metadata

Published 2023-03-23 by Kevin Feasel

Aaron Bertrand keeps metadata in sync:

As much as we tell people to use SCHEMABINDING and avoid SELECT *, there is still a wide range of reasons people do not. A well-documented problem with SELECT * in views, specifically, is that the system caches the metadata about the view from the time the view was created, not when the view is queried. If the underlying table later changes, the view doesn’t reflect the updated schema without refreshing, altering, or recreating the view. Wouldn’t it be great if you could stop worrying about that scenario and have the system automatically keep the metadata in sync?

It’s almost entirely not apropos, but the first thing I thought of when I read the title and Problem statement was Goethe’s line about Mephistopheles: “Oft evil will shall evil mar.” Make of that what you will.

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Checking XML Validity

Published 2023-03-23 by Kevin Feasel

Kevin Wilkie doesn’t like misshapen XML data:

Sometimes you’ll find that you will have XML in your database. This could be for various reasons – from storing the XML after receiving an API response to keeping it in a table because a web developer couldn’t figure out another way to store their data. Sometimes – no matter how much you trust your source – you should question if the XML is well-formed. Let’s work out a few ways you can do that in a database.

Read on for a few tests. The more concerned you are about XML data quality, the more you’d want to push in the direction of having an XSLT defined as well.

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Analyzing SQL Server Table Metadata

Published 2023-03-23 by Kevin Feasel

Barney Lawrence looks at details about a table:

For a while I’ve been building up a script that uses SQL Servers INFORMACTION_SCHEMA.COLUMNS table to create a query that applies aggregates to each column in a table and then formats the results to allow an at a glance summary of the shape of the data inside of it.

I showed some of the techniques used to build this in a session regarding metadata and data warehousing which I co-presented with Emma Dolling and Ruth Pearson at SQLBits. The one comment that I heard the most after this session was “I want that script”. I did promise to make it available more than a few times and so I’ve put together a github repository to hold it and other resources we built for the session.

Barney anticipated the most likely change I’d make while reviewing the script: APPROX_COUNT_DISTINCT() if you’re on SQL Server 2022 and dealing with a large table.

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Mnemonics for Remembering SQL Clause Order

Published 2023-03-22 by Kevin Feasel

Bob Pusateri keeps it all straight in his head:

Ooh! A mnemonic! And a pretty good one at that. The idea being that the first letter of each word of the sentence helps you remember something else, like the order of the major parts of a SELECT statement:

SELECT
FROM
WHERE
GROUP BY
HAVING
ORDER BY

Click through for Bob’s best attempts. Oddly enough, now I want some Rally’s fries. Or maybe Checkers—I can’t decide.

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Building Your Own TRY_PARSE Function

Published 2023-03-10 by Kevin Feasel

Aaron Bertrand gives it a go:

A while back, I wrote a couple of tips about simulating TRY_CONVERT – a function added in SQL Server 2012 – in earlier versions (see Part 1 and Part 2). Recently, someone in the community had a similar requirement: they wanted to make TRY_PARSE work for dates in SQL Server 2008. Now, I’m not one to help people stay on versions of SQL Server that are now 15 years old, but I do acknowledge that hands may be tied, and they can’t always control the version they’re stuck with.

Aaron does this community member a solid, though it’s probably something you’d never want to use if you’re on SQL Server 2012 or later.

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Handling Optional T-SQL Code via SQLCMD

Published 2023-03-07 by Kevin Feasel

Louis Davidson shows off a bit of SQL Server Management Studio’s SQLCMD mode:

In the creating a database script, there are quite a few parameters you will want as part of your script. The database name, the settings, whether or not to drop the database or not. Built into SQL Server Management Studio is a cool tool called SQLCMD mode. Scripts using this mode get a few scripting tools that are really useful. These tools allow you to do things like insert other script files, set environment variables, and one I particularly like, make sure you don’t accidentally execute a script if no code is highlighted using EXIT to start your script ().

Where it is really lacking is in the area of control of flow language. In this blog I am going to share a few techniques I have used to get around this when building scripts that need to optionally execute different bits of code.

Click through to see how you can use it. Going one step further, you can directly script against sqlcmd.exe, which can be useful for automating deployments.

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Approximate Percentiles in Azure SQL DB and MI

Published 2023-03-02 by Kevin Feasel

Balmukund Lakhani announces a feature has gone generally available:

Today, we are announcing General Availability (GA) of native implementation of APPROX_PERCENTILE in Azure SQL Database and Azure SQL Managed Instance. We announced preview of these functions in October 2022. Since then, many customers have adopted these for the applications where response time of percentile calculation was more important than the accuracy of the result.

I have and will continue to extol the virtues of these two functions wherever I go. They’re considerably better than the originals once you start getting into the hundreds of thousands or millions of rows. They’re also available in SQL Server 2022.

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Fun with Implicit Conversions to DateTime

Published 2023-03-01 by Kevin Feasel

Andrea Allred gets tested:

I have been teaching a T-SQL 101 class and for the homework, we asked the students to get all the records where our heroes had a birthdate between 1995 through 1999. I expected something like this:

[…]

Imagine my surprise when one of the students turned in this:

SELECT FirstName, LastName, Birthdate
FROM Heroes
WHERE Birthdate BETWEEN '1995' AND '1999'

When I first saw the query I thought, “There is no way they ran that and it worked.” So I wrote it up and ran it on my data. Guess what? IT RUNS AND RETURNS DATA! I was shocked.

Click through to see what it returns and how that’s not quite right.

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Speeding Up Queries via IF EXISTS

Published 2023-03-01 by Kevin Feasel

Chad Callihan doesn’t need to wait for the query’s end credits sequence:

When checking for the existence of a value or values in a table, typically, the whole table does not need to be read. The goal is to obtain more of a true or false answer, whether a criteria is met or not. If the criteria is met with the first few records then there’s no need to read on. Nonetheless, you may come across scripts written to unnecessarily retrieve the complete count of a value in a table. Let’s compare the performance difference between using COUNT(*) and using “IF EXISTS” when checking a table for values.

One’s going to give you a full scan and the other will give you a semi-join. Read on to see what the practical effect of this is.

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Performing a Detailed Code Review of a Stored Procedure

Published 2023-02-27 by Kevin Feasel

Aaron Bertrand has 99 problems and this stored procedure is 40 of them:

I’ve been at this for a while now, and have a very particular set of rules and coding conventions that I follow when writing and, more importantly, reviewing T-SQL code. I often perform code reviews and thought it would be fun to frame this exercise: a completely fictitious stored procedure hits my desk, I’ll reject it of course, and enumerate the reasons why. The procedure might pass review in a lot of shops, but not in mine.

Click through and give it a try. Aaron has outdone himself with this and I got angry with him about 2/3 of the way through the procedure. That’s how you know it’s a good example.

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