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Consider the following design:

CREATE TABLE dbo.Farmers ( FarmerName varchar(10) NOT NULL PRIMARY KEY ); CREATE TABLE dbo.FarmEquipment ( FarmEquipmentName varchar(10) NOT NULL PRIMARY KEY , CreatorFarmer varchar(10) NOT NULL FOREIGN KEY REFERENCES dbo.Farmers(FarmerName) ON UPDATE CASCADE ON DELETE CASCADE ); CREATE TABLE dbo.Fields ( FieldName varchar(10) NOT NULL , OwnerFarmer varchar(10) NOT NULL FOREIGN KEY REFERENCES dbo.Farmers(FarmerName) ON UPDATE CASCADE ON DELETE CASCADE , FarmEquipment varchar(10) NOT NULL FOREIGN KEY REFERENCES dbo.FarmEquipment(FarmEquipmentName) ON UPDATE CASCADE ON DELETE CASCADE );

Use case:

  • Farmer Joe owns land.
  • Farmer Ted owns land.
  • Farmer Joe builds a combine-harvester, and loans it to Farmer Ted for use on one of Farmer Ted's fields.
  • If Farmer Joe changes his name to Farmer Joseph, I want that to reflect in the Fields table, so Farmer Ted knows who owns that combine harvester he's been using.

I realize this is the exact use-case for surrogate keys, but I'm trying to determine how this would work if you're using only natural keys.

In SQL Server, you cannot actually create this structure, since the ON UPDATE CASCADE and ON UPDATE DELETE clauses in dbo.Fields produce the following error message:

Msg 1785, Level 16, State 0, Line 21
Introducing FOREIGN KEY constraint 'FK__Fields__FarmEqui__3AD6B8E2' on table 'Fields' may cause cycles or multiple cascade paths.
Specify ON DELETE NO ACTION or ON UPDATE NO ACTION, or modify other FOREIGN KEY constraints.

Msg 1750, Level 16, State 0, Line 21
Could not create constraint. See previous errors.

How should this model be expressed in SQL Server?

I realize I can just remove the ON UPDATE clauses, but that's not really the point. I want updates and deletes to cascade.

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2 Answers 2

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The solution I've seen has been to remove the foreign key constraint, and to maintain relational integrity via triggers.

On the foreign (or "child") table, you need INSERT and UPDATE triggers. if the "foreign key" column(s) are inserted/updated, then you have to make sure the value(s) exists in the primary key of the "parent" table.

On the "parent" table, on UPDATE of the primary key column(s), you have to locate all rows in any "child" tables (FarmEquipment and Fields would both be "children" of Farmers) with the old value, and update them to the new value.

On the "parent" table, on DELETE, you have to check the "child" tables to see if any rows use the primary key(s) in question, and delete them all.

Having worked with a vendor-supplied system that worked this way, I can't recommend it.

  • As noted above, you must maintain three chunks of code for each foreign key relationship, across a minimum of two triggers (let's leave self-referential situations out of the discussion, for the moment at least).
  • A "parent" table that tied to multiple "child" tables would need a check for each "child" table. While the code should be very similar for each "child" table, that can actually make modifications more difficult, as it would be easy to make a change meant to affect FarmEquipment to Fields by mistake. (In the real-world example I mentioned, there was at least one parent table with at least 20 "FK" relationships - sometimes having multiple links in the same child table, to different fields.

  • Documentation of the relationship is no longer built-in. While you can't (to the best of my knowledge) right-click a table and find all foreign key relationships that point to it, it's easy to find a query to retrieve this information from SQL Server's structural tables. When the relationship is maintained via triggers, you pretty much have to go through the triggers manually to identify all the relationships, and the precise definition (the "parent" table's DELETE trigger is where you can determine if the relationship is meant to be CASCADE, NO ACTION, SET NULL, or SET DEFAULT - with foreign key constraints, this can be seen from the "child" table).

    There are workarounds (the trigger code does exist in a queryable form), but they require that the developers follow strict rules regarding naming conventions, the construction of the query, and possibly even the formatting of the code. Even if one person is maintaining the code, and is using a template to set everything up, keeping everything exactly so as changes are made and bugs are found and fixed becomes much more difficult.

The system I worked with did work, with this method, for more than 10 years; they'd started their app in an environment that didn't have effective foreign keys. But even they had gotten to the point where they were using foreign keys for new tables, and starting to put them in place on existing tables where possible. It's just that working with it wasn't easy.

I suspect this wouldn't meet your criteria; however, as it is a viable solution, I thought it was at least worth covering.

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The design I'm going to use consists of using triggers to control referential integrity, exactly as described by @RDFozz in his answer.

I'm adding this as an answer for reference in my other question on natural vs surrogate keys.

SET NOCOUNT OFF; IF OBJECT_ID(N'dbo.Fields', N'U') IS NOT NULL DROP TABLE dbo.Fields; IF OBJECT_ID(N'dbo.FarmEquipment', N'U') IS NOT NULL DROP TABLE dbo.FarmEquipment; IF OBJECT_ID(N'dbo.Farmers', N'U') IS NOT NULL DROP TABLE dbo.Farmers; GO CREATE TABLE dbo.Farmers ( FarmerName varchar(30) NOT NULL PRIMARY KEY , RV rowversion ); CREATE TABLE dbo.FarmEquipment ( FarmEquipmentName varchar(30) NOT NULL PRIMARY KEY , CreatorFarmer varchar(30) NOT NULL , RV rowversion ); CREATE TABLE dbo.Fields ( FieldName varchar(30) NOT NULL PRIMARY KEY , OwnerFarmer varchar(30) NOT NULL , FarmEquipment varchar(30) NOT NULL , RV rowversion ); GO CREATE NONCLUSTERED INDEX IX_FarmEquipment_CreatorFarmer ON dbo.FarmEquipment(CreatorFarmer); CREATE NONCLUSTERED INDEX IX_Fields_OwnerFarmer ON dbo.Fields(OwnerFarmer); CREATE NONCLUSTERED INDEX IX_Fields_FarmEquipment ON dbo.Fields(FarmEquipment); GO CREATE TRIGGER Farmers_Update ON dbo.Farmers INSTEAD OF UPDATE AS BEGIN UPDATE dbo.Farmers SET Farmers.FarmerName = i.FarmerName FROM inserted i INNER JOIN deleted d ON i.RV = d.RV WHERE Farmers.FarmerName = d.FarmerName; UPDATE dbo.FarmEquipment SET FarmEquipment.CreatorFarmer = i.FarmerName FROM inserted i INNER JOIN deleted d ON i.RV = d.RV WHERE FarmEquipment.CreatorFarmer = d.FarmerName; UPDATE dbo.Fields SET Fields.OwnerFarmer = i.FarmerName FROM inserted i INNER JOIN deleted d ON i.RV = d.RV WHERE Fields.OwnerFarmer = d.FarmerName; END GO CREATE TRIGGER Farmers_Delete ON dbo.Farmers INSTEAD OF DELETE AS BEGIN SET NOCOUNT ON; DECLARE @farmEquipmentCount int; DECLARE @fieldsCount int; SET @farmEquipmentCount = ( SELECT COUNT(1) FROM dbo.FarmEquipment fe INNER JOIN deleted d ON fe.CreatorFarmer = d.FarmerName ); SET @fieldsCount = ( SELECT COUNT(1) FROM dbo.Fields f INNER JOIN deleted d ON f.OwnerFarmer = d.FarmerName ) IF @farmEquipmentCount = 0 AND @fieldsCount = 0 BEGIN DELETE dbo.Farmers FROM dbo.Farmers f INNER JOIN deleted d ON f.RV = d.RV; END ELSE BEGIN DECLARE @msg nvarchar(1000); SET @msg = N'Cannot remove rows from dbo.Farmers since referenced rows exist in dbo.FarmEquipment or dbo.Fields'; RAISERROR (@msg, 10, 1); ROLLBACK TRANSACTION END END GO CREATE TRIGGER FarmEquipmentInsert ON dbo.FarmEquipment INSTEAD OF INSERT AS BEGIN SET NOCOUNT ON; DECLARE @farmerCount int; SET @farmerCount = ( SELECT COUNT(1) FROM dbo.Farmers INNER JOIN inserted on Farmers.FarmerName = inserted.CreatorFarmer ); IF @farmerCount > 0 BEGIN INSERT INTO dbo.FarmEquipment (FarmEquipmentName, CreatorFarmer) SELECT inserted.FarmEquipmentName , inserted.CreatorFarmer FROM inserted; END END GO CREATE TRIGGER FarmEquipmentUpdate ON dbo.FarmEquipment INSTEAD OF UPDATE AS BEGIN SET NOCOUNT ON; UPDATE dbo.FarmEquipment SET FarmEquipment.FarmEquipmentName = i.FarmEquipmentName FROM inserted i INNER JOIN deleted d ON i.RV = d.RV WHERE dbo.FarmEquipment.RV = d.RV; UPDATE dbo.Fields SET Fields.FarmEquipment = i.FarmEquipmentName FROM inserted i INNER JOIN deleted d ON i.RV = d.RV WHERE Fields.FarmEquipment = d.FarmEquipmentName; END GO CREATE TRIGGER FarmEquipmentDelete ON dbo.FarmEquipment INSTEAD OF DELETE AS BEGIN SET NOCOUNT ON; DECLARE @FieldsCount int; SET @FieldsCount = ( SELECT COUNT(1) FROM dbo.Fields INNER JOIN deleted ON Fields.FarmEquipment = deleted.FarmEquipmentName ); IF @FieldsCount = 0 BEGIN DELETE FROM dbo.FarmEquipment FROM dbo.FarmEquipment INNER JOIN deleted ON FarmEquipment.RV = deleted.RV; END ELSE BEGIN DECLARE @msg nvarchar(1000); SET @msg = N'Cannot remove rows from dbo.FarmEquipment since referenced rows exist in dbo.Fields'; RAISERROR (@msg, 10, 1); ROLLBACK TRANSACTION END END GO CREATE TRIGGER FieldsInsertUpdate ON dbo.Fields INSTEAD OF INSERT, UPDATE AS BEGIN SET NOCOUNT ON; DECLARE @IsUpdate bit; SET @IsUpdate = CASE WHEN (SELECT COUNT(1) FROM deleted) > 0 THEN 1 ELSE 0 END; DECLARE @FarmerCount int; DECLARE @EquipmentCount int; SET @FarmerCount = ( SELECT COUNT(1) FROM dbo.Farmers INNER JOIN inserted ON Farmers.FarmerName = inserted.OwnerFarmer ); SET @EquipmentCount = ( SELECT COUNT(1) FROM dbo.FarmEquipment INNER JOIN inserted ON FarmEquipment.FarmEquipmentName = inserted.FarmEquipment ); IF @FarmerCount > 0 AND @EquipmentCount > 0 BEGIN IF @IsUpdate = 1 BEGIN UPDATE dbo.Fields SET Fields.FarmEquipment = inserted.FarmEquipment , Fields.FieldName = inserted.FieldName , Fields.OwnerFarmer = inserted.OwnerFarmer FROM inserted INNER JOIN deleted ON inserted.RV = deleted.RV WHERE deleted.RV = Fields.RV; END ELSE BEGIN INSERT INTO dbo.Fields (FieldName, FarmEquipment, OwnerFarmer) SELECT inserted.FieldName , inserted.FarmEquipment , inserted.OwnerFarmer FROM inserted; END END ELSE BEGIN DECLARE @msg nvarchar(1000); IF @IsUpdate = 1 SET @msg = N'Cannot update rows in dbo.Fields without referenced values present in dbo.Farmers and dbo.FarmEquipment.' ELSE SET @msg = N'Cannot insert rows into dbo.Fields without referenced values present in dbo.Farmers and dbo.FarmEquipment.'; RAISERROR (@msg, 10, 1); ROLLBACK TRANSACTION; END END GO INSERT INTO dbo.Farmers (FarmerName) VALUES ('Joe') , ('Ted'); INSERT INTO dbo.FarmEquipment (CreatorFarmer, FarmEquipmentName) VALUES ('Joe', 'Combine Harvester') , ('Joe', 'Tractor'); INSERT INTO dbo.Fields (OwnerFarmer, FieldName, FarmEquipment) VALUES ('Ted', 'Field 1', 'Combine Harvester') , ('Joe', 'Field 2', 'Tractor'); UPDATE dbo.Farmers SET Farmers.FarmerName = 'Joseph' WHERE Farmers.FarmerName = 'Joe'; DELETE FROM dbo.Farmers WHERE Farmers.FarmerName = 'Joseph'; UPDATE dbo.Fields SET Fields.FarmEquipment = 'Blah' WHERE Fields.FarmEquipment = 'Tractor'; UPDATE dbo.Fields SET Fields.FieldName = 'Field 3' WHERE Fields.FieldName = 'Field 2'; DELETE FROM dbo.FarmEquipment; DELETE FROM dbo.Fields; DELETE FROM dbo.Farmers; SELECT * FROM dbo.Farmers; SELECT * FROM dbo.FarmEquipment; SELECT * FROM dbo.Fields;
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