Constraint on table to limit number of records to be stored

Question

I have a database and there are two tables Ads and Images. There is a primary key adid in Ads table which is a foreign ke

Answer 1

The foreign key constraint doesn't have such option to limit the number of child rows.

It is possible to achieve the desired effect using a trigger, but this kind of constraint is relatively simple and it is possible to achieve the desired effect using only declarative constraints.

Create a helper table CheckFiveRows with column ID that contains five rows with values 1 to 5. ID is the primary key. Make sure that it contains only five rows. Add a column CheckFiveRowID to the Images table that is a foreign key pointing to CheckFiveRows.ID. Add unique constraint/index on (AdID, CheckFiveRowID).

Table Ads

CREATE TABLE [dbo].[Ads](
    [AdID] [int] NOT NULL,
    [AdData] [nvarchar](50) NOT NULL,
 CONSTRAINT [PK_Ads] PRIMARY KEY CLUSTERED 
(
    [AdID] ASC
))

Table CheckFiveRows

CREATE TABLE [dbo].[CheckFiveRows](
    [ID] [int] NOT NULL,
 CONSTRAINT [PK_CheckFiveRows] PRIMARY KEY CLUSTERED 
(
    [ID] ASC
))
GO

INSERT INTO [dbo].[CheckFiveRows] ([ID]) VALUES
(1),(2),(3),(4),(5);

Table Images

CREATE TABLE [dbo].[Images](
    [ImageID] [int] NOT NULL,
    [AdID] [int] NOT NULL,
    [CheckFiveRowID] [int] NOT NULL,
    [ImageData] [nvarchar](50) NOT NULL,
 CONSTRAINT [PK_Images] PRIMARY KEY CLUSTERED 
(
    [ImageID] ASC
))
GO

ALTER TABLE [dbo].[Images]  WITH CHECK 
ADD  CONSTRAINT [FK_Images_Ads] FOREIGN KEY([AdID])
REFERENCES [dbo].[Ads] ([AdID])
GO

ALTER TABLE [dbo].[Images] CHECK CONSTRAINT [FK_Images_Ads]
GO

ALTER TABLE [dbo].[Images]  WITH CHECK 
ADD  CONSTRAINT [FK_Images_CheckFiveRows] FOREIGN KEY([CheckFiveRowID])
REFERENCES [dbo].[CheckFiveRows] ([ID])
GO

ALTER TABLE [dbo].[Images] CHECK CONSTRAINT [FK_Images_CheckFiveRows]
GO

CREATE UNIQUE NONCLUSTERED INDEX [IX_CheckFiveRows] ON [dbo].[Images]
(
    [AdID] ASC,
    [CheckFiveRowID] ASC
)
GO

Testing

Populate Ads:

INSERT INTO [dbo].[Ads] ([AdID],[AdData]) VALUES
(1, 'a1'),
(2, 'a2');

(2 row(s) affected)

Populate Images:

INSERT INTO [dbo].[Images] ([ImageID],[AdID],[CheckFiveRowID],[ImageData]) VALUES
(1,1,1,'i1'),
(2,1,2,'i2'),
(3,1,3,'i3'),
(4,1,4,'i4');

(4 row(s) affected)

Try to add two more row for AdID=1:

INSERT INTO [dbo].[Images] ([ImageID],[AdID],[CheckFiveRowID],[ImageData]) VALUES
(5,1,5,'i5'),
(6,1,5,'i6');

Msg 2601, Level 14, State 1, Line 8
Cannot insert duplicate key row in object 'dbo.Images' with unique index 'IX_CheckFiveRows'. The duplicate key value is (1, 5).
The statement has been terminated.

Try to insert 6 rows for AdID=2:

INSERT INTO [dbo].[Images] ([ImageID],[AdID],[CheckFiveRowID],[ImageData]) VALUES
(11,2,1,'i1'),
(12,2,2,'i2'),
(13,2,3,'i3'),
(14,2,4,'i4'),
(15,2,5,'i5'),
(16,2,6,'i6');

Msg 547, Level 16, State 0, Line 13
The INSERT statement conflicted with the FOREIGN KEY constraint "FK_Images_CheckFiveRows". The conflict occurred in database "AdventureWorks2014", table "dbo.CheckFiveRows", column 'ID'.
The statement has been terminated.