Add a referential constraint with a constant value - sql

Question

I have a table of locations:
CREATE TABLE [dbo].[loca_location] (
[loca_id] [int] NOT NULL IDENTITY,
[loca_name] [nvarchar](100) NOT NULL,
[loca_address_line_1] [nvarchar](100),
[loca_address_line_2] [nvarchar](100),
[loca_address_line_3] [nvarchar](100),
[loca_address_town] [nvarchar](100),
[loca_address_county] [nvarchar](100),
[loca_post_code] [nvarchar](12),
[loca_active] [bit] NOT NULL,
[loca_created] [datetimeoffset](0) NOT NULL,
[loca_created_by] [nvarchar](50) NOT NULL,
[loca_modified] [datetimeoffset](0) NOT NULL,
[loca_modified_by] [nvarchar](50) NOT NULL,
[loca_deleted] [datetimeoffset](0),
[loca_deleted_by] [nvarchar](50),
[loca_type] [char](1),
CONSTRAINT [PK_dbo.loca_store] PRIMARY KEY ([loca_id])
)
Now some locations may be my own, others may be customers. If customer, then loca_type will be C. If I own it, it will be L. If its a supplier, then it will be S.
Now a contract should always belong to a customer, so I want to make it so the loca_type = C is a constraint as well as the loca_id.
ALTER TABLE [dbo].[cont_contract] ADD CONSTRAINT [FK_dbo.cont_contract_dbo.loca_location_cust_id] FOREIGN KEY ([loca_id_customer]) REFERENCES [dbo].[loca_location] ([loca_id], `C`)
That doesn't work. Is it possible to do what I want?

Answer 1

This may work
1.Declare UNIQUE ([loca_id], [loca_type] ) on [loca_location] table. That is correct as any superset of PK is unique.
2.Add constant column to [cont_contract] table.
3.Create FK.
CREATE TABLE [dbo].[loca_location] (
[loca_id] [int] NOT NULL IDENTITY,
--..
[loca_type] [char](1) CHECK ([loca_type] IN ('A','B','C')), -- change as needed
CONSTRAINT [PK_dbo.loca_store] PRIMARY KEY ([loca_id])
)
ALTER TABLE [dbo].[loca_location] ADD CONSTRAINT [u1] UNIQUE ([loca_id],[loca_type]);
CREATE TABLE [dbo].[cont_contract] (
[id] [int] NOT NULL IDENTITY,
--..
[loca_id_customer] Int,
-- Constant column
[loca_type] [char](1) DEFAULT 'C' CHECK ([loca_type] ='C')
);
ALTER TABLE [dbo].[cont_contract] ADD CONSTRAINT [FK_dbo.cont_contract_dbo.loca_location_cust_id]
FOREIGN KEY ([loca_id_customer],[loca_type]) REFERENCES [dbo].[loca_location] ([loca_id], [loca_type]);