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Category: Syntax

Good Use Cases for Window Functions

Published 2023-11-15 by Kevin Feasel

Aaron Bertrand build a list:

When I first used window functions back in SQL Server 2005, I was in awe. I had always used inefficient self-joins to calculate things like running totals, and these really didn’t scale well with size-of-data. I quickly realized you could also use them for ranks and moving averages without those cumbersome self-joins, elaborate sub-queries, or #temp tables. Those all have their place, but window functions can make them feel old-school and dirty.

I’d also recommend learning more about the APPLY operator as well, as it can, depending on the circumstances, be even more effective than window functions (combined with common table expressions) for some of the use cases.

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Unpivoting Data via CROSS APPLY

Published 2023-11-09 by Kevin Feasel

Aaron Bertrand’s speaking my language:

I’ve written about UNPIVOT before – see Use SQL Server’s UNPIVOT operator to help normalize output and Use SQL Server’s UNPIVOT operator to dynamically normalize output. It’s a powerful language feature that lets you flip results sideways, sort of like the opposite of PIVOT. This is great if you have columns like Phone1 and Phone2 but want to collapse them into one column. One of the challenges is that if you have other columns you also want to collapse similarly, like Email1 and Email2, you need to add an additional UNPIVOT operator or use a different approach. Is there a way to do this in a less complicated way?

Click through for the answer. And a free Curated SQL protip: if you ever want linked here, write an article about a good use case for the APPLY operator. I’m a sucker for those.

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Using the NTILE() Window Function

Published 2023-11-09 by Kevin Feasel

Joe Celko explains how NTILE() works:

The SQL NTILE() is a window function that allows you to break a table into a specified number of approximately equal groups, or <bucket count>. For each row in a grouping, the NTILE() function assigns a bucket number representing the group to which the row belong starting at one.

Click through for the explanation, including what it is and isn’t, as well as several examples.

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Getting View Definitions

Published 2023-11-08 by Kevin Feasel

Chad Callihan finds the DDL for a view:

I recently faced a situation where I had to track down the definition of a particular SQL view for various databases. I didn’t want to click through each database in SSMS to gather the information. I thought I would write a query that I could use to save some clicks and gather what I needed for each database.

The first part of that task, querying for the view definition, may be a bit tricky. I would venture to guess it may not be in the first couple of places you would think to look. Let’s walk through how we can use a query to retrieve the definition of a view.

Just make sure that you have newline retention on or else your view definition is all going on one line.

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Reasons to Prefer apply() over For Loops in R

Published 2023-11-07 by Kevin Feasel

Hugo Gruson draws some comparisons:

The debate regarding the use of for loops versus the apply() function family (apply(), lapply(), vapply(), etc., along with their purrr counterparts: map(), map2(), map_lgl(), map_chr(), etc.), has been a longstanding one in the R community.

While you may occasionally hear that for loops are slower, this notion has already been debunked in other posts. When utilized correctly, a for loop can achieve performance on par with apply() functions.

However, there are still lesser-known reasons to prefer apply() functions over for loops, which we will explore in this post.

Read on for an important caveat, and then several reasons to prefer apply() (or purrr’s counterparts). H/T R-Bloggers.

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Adding a Foreign Key while Creating a Table

Published 2023-10-31 by Kevin Feasel

Steve Jones points out one of the changes to T-SQL I really like:

This assumes I’ve added a table called dbo.Order with a PK of OrderID.

However, I can do this in the CREATE TABLE statement, like shown below. I add a new section after a column with the CONSTRAINT keyword. Then I name the constraint, which is always a good practice. I can then add the FK keyword, the column and the references that connects this child column to the parent column.

This came about in SQL Server 2014, along with In-Memory OLTP and the ability to create indexes inline with the table create script. It’s a minor quality of life thing but I do enjoy it.

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CAST() and CONVERT() for Dates

Published 2023-10-25 by Kevin Feasel

Chad Callihan converts a date:

CAST and CONVERT can both be used to switch a value to a new data type. They are similar, but certainly not identical. While CAST is considered ANSI SQL and will get you across the finish line, CONVERT can give you more flexibility when it comes to formatting date values. Let’s look at an example comparing the usage of CAST and CONVERT with dates.

Most of the time, I’ll use CAST() over CONVERT(), not so much because the former is ANSI compliant, but rather because I think it’s more intuitive to remember. Date formatting is one of the few occasions in which I usually prefer CONVERT() and that’s precisely because of the format options. Of course, if you want more custom formatting options, you can use FORMAT(), though that function uses .NET in the background and is remarkably slow. It’s fine if you’re formatting a few dates, but if you’re outputting millions of rows, you will certainly see a marked difference.

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Substrings in MySQL

Published 2023-10-16 by Kevin Feasel

Rahul Mehta describes how substrings work in MySQL:

MySQL is an open-source relational database management system. It is a widely used relational database management system in the entire world. MySQL like any other database can store different types of data. One of the most used data types is “String”. Developers widely use it in storing data as well as in different formatting operations. One of the key requirements we will always come across is to derive a part of the string. MySQL provides a “SUBSTRING” function to extract a substring from a string. My SQL has below options for extracting the string:

  1. SUBSTRING
  2. SUBSTR (A SYNONYM FOR SUBSTRING)
  3. SUBSTRING_INDEX

Read on to see how these two functions work. They differ a bit from SQL Server in terms of functionality, though there’s a lot of overlap between the two.

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MERGE is (Kinda) Okay

Published 2023-10-03 by Kevin Feasel

Hugo Kornelis performs a survey:

The MERGE statement compares source and target data, and then inserts into, updates, and deletes from the target table, all in a single statement. This statement was introduced in SQL Server 2008. I liked it, because it allows you to replace a set of multiple queries with just one single query. And while a statement with that many options necessarily has a more complex syntax, I still believe that, in most cases, a single MERGE statement is easier to read, write, and maintain, than a combination of at least an INSERT and an UPDATE, often a DELETE, and sometimes first a SELECT into a temporary table if the source is complex.

Click through for a review of a variety of problems people have had in the past. It surprised me a bit when I learned how few of these issues were still active problems caused by MERGE.

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