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Category: Constraints

Finding SQL Server Columns with Defaults

Tom Collins sticks to the defaults:

Do you have a sql query to check every  sql server database  column and identify if a default value is applied to the column?  

Click through for a script which does just that. Tom’s query goes against system views and there’s a separate way to get those details from sys.default_constraints if you prefer to have a second option. If you’re on an older version of SQL Server where CONCAT_WS() doesn’t exist, concatenate it yourself.

SELECT	CONCAT_WS('.', QUOTENAME(OBJECT_SCHEMA_NAME(c.object_id)), QUOTENAME(OBJECT_NAME(c.object_id))) AS TableName, AS ColumnName, AS DefaultConstraintName,	dc.definition AS DefaultConstraintDefinition
FROM sys.default_constraints dc	INNER JOIN sys.columns c	ON dc.parent_object_id = c.object_id	AND dc.parent_column_id = c.column_id;
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The Importance of Naming Constraints in SQL Server

Eitan Blumin gives everything a name:

This article was published by Aaron Bertrand a few years ago, talking about system-named constraints in SQL Server.

The article mostly focuses on the issue of naming conventions as the main issue with system-named constraints and provides a useful stored procedure script to generate sp_rename commands for all system-named constraints.

However, the script in the article provides the solution for only one database and doesn’t support the new “Edge Constraints” that were introduced in SQL Server 2019.

Check out Aaron’s article and Eitan’s follow-up piece.

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Database Constraints in Postgres

Grant Fritchey does some data modeling:

PostgreSQL supports constraints much like any other database management system. When you need to ensure certain behaviors of the data, you can put these constraints to work. I’ve already used several of these in creating my sample database (available articles publicly on GitHub, in the CreateDatabase.sql file). I’ll explain those as we go. The constraints supported by PostgreSQL are:

Read on for the list, which includes one constraint which doesn’t have a direct analog in SQL Server.

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Defending (Certain) Bad Practices

Aaron Bertrand considers the trade-offs:

For the first T-SQL Tuesday in 2023, Raul Gonzalez invites us to talk about cases where we have knowingly implemented worst practices.

Well, I have done it a lot. Most of the posts in my bad habits series are cautionary tales based on my own “learning the hard way.” There are always trade-offs with doing something correctly – maybe proper design is less efficient, or takes longer to write, or has to pass more checks. Over time, though, you start getting a feel for where it makes sense to cut these corners, and where it doesn’t.

Read on for some practical examples around Stack Overflow.

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Deleting from Multiple Tables by Key

Guy Glantser needs to delete some data:

Sometimes you need to delete data from multiple tables in a database. For example, you might have a multi-tenant database, and you need to delete all the data that belong to some tenant.

The problem is that there are many tables in the database that contain data, which you need to delete. If you have a column like “TenantId” in all tables, then your life is easier, because you have a simple predicate to apply to your DELETE statements against all tables. But even then, if there are foreign keys between tables to enforce referential integrity, then things get more complicated.

Read through for Guy’s answer, which definitely works and can be the quickest solution. If you can’t drop foreign key constraints (even temporarily), I have a post from a while back on tracing foreign keys to “levels.” The post only covers finding the ordering but could be extended to delete data one level at a time.

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Join Removal Due to Foreign Key Constraint

David Alcock shows a performance benefit from having a foreign key constraint in place:

Foreign keys are used in database design to enforce referential integrity but they also have some performance benefits as well that you might not necessarily notice unless you’re looking into your execution plans.

Let’s take the following query using the AdventureWorks2019 sample database where I’m selecting the BusinessEntityID and JobTitle from the HumanResources.Employee table and by using an inner join I’m only returning rows that have matching values (BusinessEntityID) in both tables:

There are specific rules to table elimination but if you meet the criteria, it can save you a bit of CPU time and I/O.

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Determining Why Constraints Are Untrusted

Tom Zika adopts a zero-trust constraint architecture:

Okay, so you went through the effort of fixing them, but the next day your constraints are not trusted again. What gives?

If you are sure none of the DBAs or developers is doing this to spite you, the most common culprit is a BULK INSERT or bulk copy tool (bcp).

One of its parameters is -h(hints)
and one of those hints is CHECK_CONSTRAINTS

Read on to see how this can mess everything up, as well as how you can track and fix it. There are cases, particularly in extremely high-write systems, where you don’t necessarily need the constraint to exist but want it to be there for documentation purposes. In that case, the constraints are usually disabled rather than simply untrusted. The other time I see people purposefully using untrusted constraints is that old data is garbage and essentially unfixable but they want new data to be correct. Most of the time, though, constraints are untrusted because nobody noticed the problem.

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