Cascading Deletes in VS .NET

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Usually, referential integrity between two tables is implemented with a foreign key, such as in the following illustration:
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In such cases, a foreign key prevents the user from deleting records from a parent table (Inventory) if a record is referenced in a linked table (InventoryProperty). The only way to delete the record would be to use the following code:
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Delete InventoryProperty Where InventoryId = 222 Delete Inventory Where InventoryId = 222
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In some cases, the system design requirements might call for cascading deletes, which automatically delete records from the linked table when the record in the parent table is deleted. In this case, only one command is required to delete any instance of an asset with an InventoryId value of 222:
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Delete Inventory Where InventoryId = 222
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SQL Server 2000 introduces cascading referential integrity constraints that can implement this behavior. In SQL Server 7.0 and earlier versions, you had to use triggers to implement cascading operations. The following example creates two new tables (without a foreign key), populates them with a few records, and creates a trigger that will implement a cascading delete:
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Create Table MyInventory ( Inventoryid int Not Null Identity (1, 1), EquipmentId int Null, LocationId int Null, StatusId tinyint Null, LeaseId int Null, LeaseScheduleId int Null, OwnerId int Null, Rent smallmoney Null, Lease smallmoney Null, Cost smallmoney Null, AcquisitionTypeID tinyint Null ) Go Alter Table MyInventory Add Constraint PK_Inventory Primary Key Nonclustered (
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Inventoryid ) Go Create Table MyInventoryProperty ( InventoryId int Not Null, PropertyId smallint Not Null, Value varchar(50) Null ) Go Alter Table MyInventoryProperty Add Constraint PK_InventoryProperty Primary Key Nonclustered ( InventoryId, PropertyId ) Go Create Trigger trMyInventory_CascadingDelete_D On MyInventory After Delete --For delete As If @@Rowcount = 0 Return Delete MyInventoryProperty where InventoryId In (Select InventoryID from deleted) Go Insert Insert Insert Insert Insert Insert Values Insert Values Insert Values Insert Values into into into into into myInventory(EquipmentId) myInventory(EquipmentId) myInventory(EquipmentId) myInventory(EquipmentId) myInventory(EquipmentId) Values Values Values Values Values (1) (2) (3) (4) (5) PropertyId, Value) PropertyId, Value) PropertyId, Value) PropertyId, Value)
into myInventoryProperty(InventoryId, (1, 1, 'ACME') into myInventoryProperty(InventoryId, (1, 2, 'Turbo') into myInventoryProperty(InventoryId, (1, 3, '311') into myInventoryProperty(InventoryId, (2, 1, 'ACME')
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into myInventoryProperty(InventoryId, PropertyId, Value) (2, 2, 'TurboPro') into myInventoryProperty(InventoryId, PropertyId, Value) (2, 3, '312')
Delete MyInventory Where InventoryId = 1 Select * from myInventory Select * from myInventoryProperty
Aggregates
Imagine that users of an Asset database are often clogging the Inventory table. One operation that they perform often is the execution of reports that prepare the sum of all monthly lease payments per lease schedule. If the sum were prepared in advance, the report would be available in an instant, the table would be less clogged, and the user would experience fewer locking and deadlocking problems. To provide this functionality, you could create one or more triggers to maintain the PeriodicTotalAmount field in the LeaseSchedule table. The field will contain the sum of lease payments for assets in the Inventory table that are associated with a lease schedule. It is possible to implement diverse solutions for this task. This solution is based on separate triggers for different modification statements. The Insert trigger is based on a relatively complex Update statement with a subquery based on the contents of the Inserted table. Each new record increments the total in the related lease schedule. The Coalesce statement is used to replace nulls with zeros in the calculation. The trigger evaluates the number of records affected by the modification statement at the beginning and, if no records are affected, aborts further execution. This solution executes properly even when multiple records are inserted with one statement:
Create Trigger trInventory_Lease_I On dbo.Inventory after Insert -- For Insert As If @@Rowcount = 0 return
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-- add inserted leases to total amount Update LeaseSchedule Set LeaseSchedule.PeriodicTotalAmount = LeaseSchedule.PeriodicTotalAmount + Coalesce(i.Lease, 0) from LeaseSchedule inner join inserted i on LeaseSchedule.ScheduleId = i.LeaseScheduleId Go
The Delete trigger is very similar to the previous trigger. The main difference is that the values from the Deleted table are subtracted from the total, as shown here:
Create Trigger trInventory_Lease_D On dbo.Inventory After Delete -- For delete As If @@Rowcount = 0 Return -- subtract deleted leases from total amount Update LeaseSchedule Set LeaseSchedule.PeriodicTotalAmount = LeaseSchedule.PeriodicTotalAmount - Coalesce(d.Lease, 0) from LeaseSchedule inner join deleted d on LeaseSchedule.ScheduleId = d.LeaseScheduleId Go
The Update trigger is the most complicated. The calculation of a total is performed only if the Lease and LeaseScheduleId fields are referenced by the Update statement. The trigger then subtracts the Lease amounts from the deleted records and adds the Lease amounts from the inserted records to the related totals.
Create Trigger trInventory_Lease_U On dbo.Inventory After Update -- For Update As if @@Rowcount = 0 return
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