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Category: Constraints

Metadata-Only NOT NULL Column Insertion

Published 2024-11-20 by Kevin Feasel

Andy Brownsword has the need for speed:

When adding a new column and wanting to default the value for existing records, it used to be a painful task. As of SQL Server 2012 that became much easier.

But nobody told me, until Simon casually mentioned it in conversation recently. I had to see it for myself, so I thought I’d share for those who weren’t aware.

Read on to see how. I rarely self-promote in other people’s blog posts (hush, person who knows all the times I’ve done it), but I do have a talk on the topic of near-zero downtime database deployment strategies which includes this and quite a few other notes on what you can do without blocking others. For these sorts of changes, what you’re looking for is asynchronous processing and a Sch-M (schema modification) lock at the very end, such as when rebuilding an index with ONLINE = ON in Enterprise Edition. Alternatively, look for a Sch-M lock only on a metadata table and not the actual data. Andy’s post is an example of the latter.

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Finding Foreign Key Constraints without Backing Indexes

Published 2024-11-04 by Kevin Feasel

Andy Brownsword goes searching:

That’s all well and good if we’re creating a new foreign key and adding our own index. How do we find existing foreign keys which aren’t yet indexed?

That’s where the query below comes into play.

We use a CTE to find foreign keys which are already indexed. From this we can then return details of other foreign keys along with scripts to create (and roll back) an index to mirror the key:

Click through for the script and notes on how it all works.

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Data Integrity and the LIKE Operator

Published 2024-08-22 by Kevin Feasel

Joe Celko provides some context:

The LIKE operator has been in SQL since the very beginning. It’s a very weak form of pattern matching, that has only two simple wildcards. The wildcards are ‘_‘ (underscore) and ‘%‘ (percent). They respectively stand for any single character and for zero or more characters in a substring. The syntax is pretty straightforward:

Read on for examples of LIKE in action, including in check constraints.

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The Power of the Check Constraint

Published 2024-05-21 by Kevin Feasel

Joe Celko defends the honor of the check constraint:

The problem is that newer SQL programmers (and even some more experienced ones) don’t take advantage of these constraints. Consider this slight rewrite of an actual posting on an SQL forum. In the original, the poster thought that identifiers should be integers and that ISO–11179 naming rules do not apply, so I cleaned it up a little, but this will make my point.

Click through to learn more about check constraints.

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Contrasting Three Unique Identifiers in Postgres

Published 2024-02-05 by Kevin Feasel

Laetitia Avrot shares some advice:

This month’s PGSQLPhriday event is about UUId thanks to my post calling for a fight debate on the topic.

I will answer a question a friend developer asked me: “What is the best when we need a primary key? UUID, CUID, or TSID?”

I hadn’t heard of two of these, but Laetitia provides some links to learn more about them and then offers up some advice on whether to use any of them. And the advice sounds a lot like the advice for SQL Server.

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An Overview of Join Elimination

Published 2024-02-02 by Kevin Feasel

Chad Callihan doesn’t need solo rows; he needs team players:

SQL Server can be pretty smart when it comes to avoiding work it’s not required to do. One instance of this is the concept of join elimination. Join elimination occurs when queries involving joins may only need to check one table instead of two (or more) based on foreign key constraints.

Let’s take a look at join elimination in action.

Read on for one example of join elimination and one more reason why those foreign key constraints are so useful.

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Foreign Key Discovery in SQL Server & Azure SQL DB

Published 2024-01-04 by Kevin Feasel

Josephine Bush walks around town with a lantern looking for a good foreign key:

There are plenty of times I’m called upon to fix data. To do this, I must know what dependencies are in the database. Foreign keys are a crucial aspect of maintaining data integrity within relational databases. They establish relationships between tables, ensuring data references remain consistent and accurate. In an Azure SQL Database, identifying and managing foreign keys is essential for maintaining a well-structured and reliable database architecture.

Click through for a primer on foreign key constraints, a few ways to find them, and some closing thoughts on working with tables containing foreign key constraints.

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The Value of Indexing Foreign Key Columns

Published 2023-12-22 by Kevin Feasel

Etienne Lopes takes us through a scenario:

Let me start this post with a question, “Do you think that it can be beneficial to have a single column index for the foreign key column in the child table?“

Well, I believe I can ear three types of answers to this question:

  • Always!
  • Never!
  • It Depends…

Click through for Etienne’s answer. I’d still prefer these indexes to have multiple uses, which generally means having enough columns on the index to act as a covering index for one or more important queries. But Etienne does show a good use case for this single-column index.

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