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Category: Constraints

The Value of Foreign Keys

Kevin Wilkie tempts Betteridge’s Law of Headlines:

Foreign Keys are truly columns that point to the primary key of another table. Yes, you can create a foreign key pointing to the same table – but that’s… well… special. There are reasons, but far beyond the reasons I want to go into foreign keys here.

What a foreign key does for you is extremely important – Enforce Data Consistency.

Click through to learn how.

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Why Have Multiple Edge Constraints in SQL Graph?

Louis Davidson has an explanation for us:

Edge constraints were added in SQL Server 2019 to make the node to edge relationship stricter/enforced, and more like typical foreign key constraints. When used, they define what node types can be used in the from and to position of the edge. What makes edges different than a many-to-many relationship in a relational table is that an edge can implement more than one many-to-many relationship in a single table. To constrain the types of data that can be put into the edge, you can use an edge constraint.

Edge constraints are very similar to implementing foreign key constraints, but there are a few key differences. Foreign keys are between two tables. Edges are between one edge table, and multiple pairs of node tables. In both cases, you can have multiple constraints, even from the same table to the same related table on the same column. However, with edge constraints, because you can have multiple pairs of expressions, and even multiple constraints, it bears discussion. If you have more than one constraint, it has one big negative, but it is allowed to implement one big positive!

Click through for the explanation, as well as an example.

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Fun with Multi-Column Unique Constraints

Aaron Bertrand has an interesting use case:

A problem that comes up occasionally with constraints in SQL Server is when a unique constraint applies to multiple columns, but the values in those columns can be populated in any order. For example, if a table holds interactions between two users, and has columns User1 and User2, there should only be one row allowed for users 100 and 200, regardless of whether the data is entered as 100, 200 or 200, 100.

Click through for one solution. Another solution would be to normalize this down further with a dbo.ConversationParticipants table.

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Foreign Keys and Updating the Parent

Hugo Kornelis conclues a mini-series on foreign key constraints:

Welcome to part fourteen of the plansplaining series, where I wrap up the mini-series on how simple foreign keys have huge effects on execution plans for data modifications.

We already looked at inserting data in the referencing (child) table, and at deleting data from the referenced (parent) table as well as updates in the child table. We did not and will not look at deleting from the child table or inserting in the parent table: those operations can by default never violate the foreign key constraint, so no additional logic is needed.

So that means there is only one thing left to explore: updating the parent. Perhaps surprisingly, this is actually quite complex, so it warrants an entire post of its own.

Read on to see why.

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Execution Plans: Foreign Keys and Deletion

Hugo Kornelis continues a series on execution plans:

In the previous part, we looked at foreign key checking when data is inserted. Inserts in the referencing (child) table can, by definition, not cause foreign key violations, so no checking is done in those cases. But for inserts in the referenced (parent) table, SQL Server needs to check that the inserted value does indeed exist. We saw how both the Nested Loops and the Merge Join operators have some specific optimizations to do that check as efficient as possible; we also saw how the Assert operator does the actual checking.

When deleting data, the reverse is true. We can delete rows from the child table without verification, but deletes from the parent table are subject to a check.

And those deletes can get crazy expensive.

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How Foreign Keys Appear in Execution Plans

Hugo Kornelis shows us how foreign key contraints modify certain execution plans:

The top left of this execution plan looks very unsurprising. A Clustered Index Scan to read all rows from the #Products temporary table, and those rows are then passed to a Clustered Index Insert operator for insertion into our permanent table dbo.Products. But wait? Why is there no table or index name listed below the operator name? That’s actually because the operator has not one but three items in its Object property. When a Clustered Index Insert targets just a single index, SSMS can work out the name and show it. But when Clustered Index Insert targets multiple objects, SSMS plays it safe and displays none.

Read on to learn more.

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Supporting Multiple NULL Values with a Unique Constraint

Itzik Ben-Gan walks us through a workaround in T-SQL:

Suppose that you have a database in a platform that supports the standard unique constraint and you need to migrate that database to SQL Server. You may face issues with the enforcement of unique constraints in SQL Server if the unique columns support NULLs. Data that was considered valid in the source system may be considered invalid in SQL Server. In the following sections I’ll explore a number of possible workarounds in SQL Server.

I use a simplified version of this as an interview question, so it’s nice to see an entire article from Itzik on the topic, including a couple solutions way outside the box.

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Finding Foreign Key Relationships in SQL Server

John Morehouse shows how you can piece together foreign key relationships in SQL Server:

Recently, I had to purge some parent records from a table.  In this case, the parent table had foreign keys, which itself isn’t an issue.  The fact that there were more than 30 of them was.   While SQL Server will happily tell you that you are violating a foreign key if a child record is present when deleting the parent record, finding all of them can be cumbersome.  This is even more true when you have a larger number of foreign keys.

Thankfully, SQL Server can tell us a lot of information about foreign keys including both the parent and child tables as well as the column used.  From this information, we can dynamically create a SELECT statement that would tell us the number of child records that are tied to the parent ID.

Click through for the solution.

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Finding Data Which Breaks Constraints

Phil Factor has a procedure to test disabled check constraints and find data which would cause an error:

However often I go on about CHECK constraints, there will always be a developer who will leave them out or mutter in a dignified manner about how all checks need to be done only at the application level. This attitude soon gets divine retribution. Bad data springs up like a rotting fungus over your database unless you add CHECK constraints to all your tables. This is fine but then how do you prevent the excellent and estimable habit of adding them to then interfere with a release? The constraints will stop the build if they meet bad data: it is what they are trained to do. If you don’t like that, then you must fix the bad data first.

Click through for the process.

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