用于检查约束的条件

Question

I have a table that holds Tasks for a particular person.

TaskID INT PK
PersonID INT (FK to Person Table)
TaskStatusID INT (FK To list of Statuses)
Deleted DATETIME NULL

The business rule is that a person can not have more than one active task at a time. A task is 'Active' based on it's TaskStatusID. The statuses are:

'5=New, 6=In 7=Progress, 8=Under 9=Review, 10=Complete, 11=Cancelled'

These are values in my Status table.

So, 5,6,7,8 and 9 are Active tasks. These rest are finalised.

A person can only have one task which is in an active state.

So, to test if I can add a task for this person, I would do:

CASE EXISTS(SELECT * FROM Task WHERE PersonID = 123 AND TaskStatusIN IN (5,6,7,8,9)) THEN 0 ELSE 1 END AS CanAdd

The table has a lot of rows. Around 200,000.

I was thinking of adding a Check Constraint on this table, so on update/insert, I make that query to see if the row being added/edited will break the data integrity with regards the business rules.

Is a check constraint suitable for this, or is there a more efficient way to keep the data integral.

Something like:

ADD CONSTRAINT chk_task CHECK (
    EXISTS(SELECT * FROM Task WHERE PersonID = ?? AND TaskStatusIN IN (5,6,7,8,9)))

Answer 1

You can't easily do it with a check constraint because they only (naturally) can make assertions about columns within the same row. There are some kludgy ways to get around that by using a UDF to query other rows but most implementations I've seen have odd edge cases where it's possible to work around the UDF and end up with invalid rows after all.

What you can do is to create an indexed view that maintains the constraint:

create table dbo.Tasks (
TaskID INT not null primary key,
PersonID INT not null,
TaskStatusID INT not null,
Deleted DATETIME NULL
)
go
create view dbo.DRI_Tasks_OneActivePerPerson
with schemabinding
as
    select PersonID from dbo.Tasks
    where TaskStatusID IN (5,6,7,8,9)
go
create unique clustered index UX_DRI_Tasks_OneActivePerPerson
on dbo.DRI_Tasks_OneActivePerPerson (PersonID)

And now this insert succeeds (because there's only one row with an active status for person 1:

insert into dbo.Tasks (TaskID,PersonID,TaskStatusID)
values (1,1,5),(2,1,1),(3,1,4)

But this insert fails:

insert into dbo.Tasks (TaskID,PersonID,TaskStatusID)
values (4,2,6),(5,2,8)

With the message:

Cannot insert duplicate key row in object 'dbo.DRI_Tasks_OneActivePerPerson'
with unique index 'UX_DRI_Tasks_OneActivePerPerson'.
The duplicate key value is (2).

Answer 2

If you are using SQL Server 2008 or later version, you could create a unique filtered index:

CREATE UNIQUE INDEX UQ_ActiveStatus
ON dbo.Task (PersonID)
WHERE TaskStatusID IN (5, 6, 7, 8, 9);

It would act as a unique constraint specifically for rows with the specified statuses. You would only be able to have one of the specified statuses per person.

Answer 3

您可以使用上面的检查约束，但是我建议在插入/更新之前编写dml触发器的最佳方法是提升语句。