SQL foreign key constraint with additional column value check - sql

Question

I have this situation
CREATE TABLE [dbo].[Customer]
(
Id INT IDENTITY(1,1) NOT NULL,
IsWholesaler BIT NOT NULL,
Name NVARCHAR(255) NOT NULL
)
CREATE TABLE [dbo].[WholesalerUser]
(
Id INT IDENTITY(1,1) NOT NULL,
CustomerId INT NOT NULL,
UserId INT NOT NULL
)
CREATE TABLE [dbo].[User]
(
Id INT IDENTITY(1,1) NOT NULL,
Name NVARCHAR(255)
)
One Customer which has IsWholesaler column set to true can have multiple users.
I have WholesalerUser table for this purpose but the Foreign key WholesalerUser.CustomerId is attached to Customer.Id but how to check the column IsWholesaler = 1 ?

Answer 1

Foreign key can be defined over several columns at once.
Add a column IsWholesaler to the WholesalerUser.
Add a check constraint to make sure that it is always 1.
Define foreign key over two columns (CustomerId, IsWholesaler).
To define such foreign key the Customer table should have either primary key as (ID, IsWholesaler), or a separate unique index on (ID, IsWholesaler).
Table Customer
CREATE TABLE [dbo].[Customer](
[Id] [int] IDENTITY(1,1) NOT NULL,
[IsWholesaler] [bit] NOT NULL,
[Name] [nvarchar](255) NOT NULL,
CONSTRAINT [PK_Customer] PRIMARY KEY CLUSTERED
(
[Id] ASC
))
Unique index to support foreign key constraint:
CREATE UNIQUE NONCLUSTERED INDEX [IX_ID_IsWholesaler] ON [dbo].[Customer]
(
[Id] ASC,
[IsWholesaler] ASC
)
Table WholesalerUser
CREATE TABLE [dbo].[WholesalerUser](
[Id] [int] IDENTITY(1,1) NOT NULL,
[CustomerId] [int] NOT NULL,
[IsWholesaler] [bit] NOT NULL,
[UserId] [int] NOT NULL,
CONSTRAINT [PK_WholesalerUser] PRIMARY KEY CLUSTERED
(
[Id] ASC
))
Foreign key
ALTER TABLE [dbo].[WholesalerUser] WITH CHECK
ADD CONSTRAINT [FK_WholesalerUser_Customer]
FOREIGN KEY([CustomerId], [IsWholesaler])
REFERENCES [dbo].[Customer] ([Id], [IsWholesaler])
GO
ALTER TABLE [dbo].[WholesalerUser]
CHECK CONSTRAINT [FK_WholesalerUser_Customer]
GO
Check constraint
ALTER TABLE [dbo].[WholesalerUser] WITH CHECK
ADD CONSTRAINT [CK_WholesalerUser]
CHECK (([IsWholesaler]=(1)))
GO
ALTER TABLE [dbo].[WholesalerUser]
CHECK CONSTRAINT [CK_WholesalerUser]
GO
ALTER TABLE [dbo].[WholesalerUser]
ADD CONSTRAINT [DF_WholesalerUser_IsWholesaler]
DEFAULT ((1)) FOR [IsWholesaler]
GO