Not directly related to the OP, but since Adrian's expressed an interest. Here's a table than SQL Server maintains the integrity of, ensuring that only one valid value is present at any time. In this case, I'm dealing with a current/history table, but the example can be modified to work with future data also (although in that case, you can't have the indexed view, and you need to write the merge's directly, rather than maintaining through triggers).
In this particular case, I'm dealing with a link table that I want to track the history of. First, the tables that we're linking:
create table dbo.Clients (
ClientID int IDENTITY(1,1) not null,
Name varchar(50) not null,
/* Other columns */
constraint PK_Clients PRIMARY KEY (ClientID)
)
go
create table dbo.DataItems (
DataItemID int IDENTITY(1,1) not null,
Name varchar(50) not null,
/* Other columns */
constraint PK_DataItems PRIMARY KEY (DataItemID),
constraint UQ_DataItem_Names UNIQUE (Name)
)
go
Now, if we were building a normal table, we'd have the following (Don't run this one):
create table dbo.ClientAnswers (
ClientID int not null,
DataItemID int not null,
IntValue int not null,
Comment varchar(max) null,
constraint PK_ClientAnswers PRIMARY KEY (ClientID,DataItemID),
constraint FK_ClientAnswers_Clients FOREIGN KEY (ClientID) references dbo.Clients (ClientID),
constraint FK_ClientAnswers_DataItems FOREIGN KEY (DataItemID) references dbo.DataItems (DataItemID)
)
But, we want a table that can represent a complete history. In particular, we want to design the structure such that overlapping time periods can never appear in the database. We always know which record was valid at any particular time:
create table dbo.ClientAnswerHistories (
ClientID int not null,
DataItemID int not null,
IntValue int null,
Comment varchar(max) null,
/* Temporal columns */
Deleted bit not null,
ValidFrom datetime2 null,
ValidTo datetime2 null,
constraint UQ_ClientAnswerHistories_ValidFrom UNIQUE (ClientID,DataItemID,ValidFrom),
constraint UQ_ClientAnswerHistories_ValidTo UNIQUE (ClientID,DataItemID,ValidTo),
constraint CK_ClientAnswerHistories_NoTimeTravel CHECK (ValidFrom < ValidTo),
constraint FK_ClientAnswerHistories_Clients FOREIGN KEY (ClientID) references dbo.Clients (ClientID),
constraint FK_ClientAnswerHistories_DataItems FOREIGN KEY (DataItemID) references dbo.DataItems (DataItemID),
constraint FK_ClientAnswerHistories_Prev FOREIGN KEY (ClientID,DataItemID,ValidFrom)
references dbo.ClientAnswerHistories (ClientID,DataItemID,ValidTo),
constraint FK_ClientAnswerHistories_Next FOREIGN KEY (ClientID,DataItemID,ValidTo)
references dbo.ClientAnswerHistories (ClientID,DataItemID,ValidFrom),
constraint CK_ClientAnswerHistory_DeletionNull CHECK (
Deleted = 0 or
(
IntValue is null and
Comment is null
)),
constraint CK_ClientAnswerHistory_IntValueNotNull CHECK (Deleted=1 or IntValue is not null)
)
go
That's a lot of constraints. The only way to maintain this table is through merge statements (see examples below, and try to reason about why yourself). We're now going to build a view that mimics that ClientAnswers table defined above:
create view dbo.ClientAnswers
with schemabinding
as
select
ClientID,
DataItemID,
ISNULL(IntValue,0) as IntValue,
Comment
from
dbo.ClientAnswerHistories
where
Deleted = 0 and
ValidTo is null
go
create unique clustered index PK_ClientAnswers on dbo.ClientAnswers (ClientID,DataItemID)
go
And we have the PK constraint we originally wanted. We've also used ISNULL to reinstate the not null-ness of the IntValue column (even though the check constraints already guarantee this, SQL Server is unable to derive this information). If we're working with an ORM, we let it target ClientAnswers, and the history gets automatically built. Next, we can have a function that lets us look back in time:
create function dbo.ClientAnswers_At (
@At datetime2
)
returns table
with schemabinding
as
return (
select
ClientID,
DataItemID,
ISNULL(IntValue,0) as IntValue,
Comment
from
dbo.ClientAnswerHistories
where
Deleted = 0 and
(ValidFrom is null or ValidFrom <= @At) and
(ValidTo is null or ValidTo > @At)
)
go
And finally, we need the triggers on ClientAnswers that build this history. We need to use merge statements, since we need to simultaneously insert new rows, and update the previous "valid" row to end date it with a new ValidTo value.
create trigger T_ClientAnswers_I
on dbo.ClientAnswers
instead of insert
as
set nocount on
;with Dup as (
select i.ClientID,i.DataItemID,i.IntValue,i.Comment,CASE WHEN cah.ClientID is not null THEN 1 ELSE 0 END as PrevDeleted,t.Dupl
from
inserted i
left join
dbo.ClientAnswerHistories cah
on
i.ClientID = cah.ClientID and
i.DataItemID = cah.DataItemID and
cah.ValidTo is null and
cah.Deleted = 1
cross join
(select 0 union all select 1) t(Dupl)
)
merge into dbo.ClientAnswerHistories cah
using Dup on cah.ClientID = Dup.ClientID and cah.DataItemID = Dup.DataItemID and cah.ValidTo is null and Dup.Dupl = 0 and Dup.PrevDeleted = 1
when matched then update set ValidTo = SYSDATETIME()
when not matched and Dup.Dupl=1 then insert (ClientID,DataItemID,IntValue,Comment,Deleted,ValidFrom)
values (Dup.ClientID,Dup.DataItemID,Dup.IntValue,Dup.Comment,0,CASE WHEN Dup.PrevDeleted=1 THEN SYSDATETIME() END);
go
create trigger T_ClientAnswers_U
on dbo.ClientAnswers
instead of update
as
set nocount on
;with Dup as (
select i.ClientID,i.DataItemID,i.IntValue,i.Comment,t.Dupl
from
inserted i
cross join
(select 0 union all select 1) t(Dupl)
)
merge into dbo.ClientAnswerHistories cah
using Dup on cah.ClientID = Dup.ClientID and cah.DataItemID = Dup.DataItemID and cah.ValidTo is null and Dup.Dupl = 0
when matched then update set ValidTo = SYSDATETIME()
when not matched then insert (ClientID,DataItemID,IntValue,Comment,Deleted,ValidFrom)
values (Dup.ClientID,Dup.DataItemID,Dup.IntValue,Dup.Comment,0,SYSDATETIME());
go
create trigger T_ClientAnswers_D
on dbo.ClientAnswers
instead of delete
as
set nocount on
;with Dup as (
select d.ClientID,d.DataItemID,t.Dupl
from
deleted d
cross join
(select 0 union all select 1) t(Dupl)
)
merge into dbo.ClientAnswerHistories cah
using Dup on cah.ClientID = Dup.ClientID and cah.DataItemID = Dup.DataItemID and cah.ValidTo is null and Dup.Dupl = 0
when matched then update set ValidTo = SYSDATETIME()
when not matched then insert (ClientID,DataItemID,Deleted,ValidFrom)
values (Dup.ClientID,Dup.DataItemID,1,SYSDATETIME());
go
Obviously, I could have built a simpler table (not a join table), but this is my standard go-to example (albeit it took me a while to reconstruct it - I forgot the set nocount on statements for a while). But the strength here is that, the base table, ClientAnswerHistories is incapable of storing overlapping time ranges for the same ClientID and DataItemID values.
Things get more complex when you need to deal with temporal foreign keys.
Of course, if you don't want any real gaps, then you can remove the Deleted column (and associated checks), make the not null columns really not null, modify the insert trigger to do a plain insert, and make the delete trigger raise an error instead.
