How do I implement a key to be both PK and FK? - sql

Question

I'm very new to SQL and just practicing for my SQL exam by going through past papers, however, I'm stuck on how to implement the staffID & projNo in the allocation table to be both primary and foreign keys. I've tried to use solutions online but none work.
Here is the relational schema (PK, FK)
staff( **staffID**, firstname, lastName, gender, dob, jobTitle)
project ( **projNo**, projName, description )
allocation ( ***staffID***, ***projNo***, hours )
Here is my SQL:
CREATE TABLE staff (
staffID CHAR (4) PRIMARY KEY,
firstName VARCHAR (30) NOT NULL,
lastName VARCHAR (30) NOT NULL,
gender CHAR (1) CHECK (gender IN ('M','F'))
dob DATE NOT NULL,
jobTitle VARCHAR (30) NOT NULL,
);
CREATE TABLE project (
projNo CHAR (4) PRIMARY KEY,
projName VARCHAR (20) NOT NULL,
description VARCHAR (30) NOT NULL,
);
CREATE TABLE allocation (
staffID CHAR (4)
projNo CHAR (4)
hours int (2)
);

Answer 1

Making the two column to have Primary Key Constraint Use;
Alter table table_name Add MyPrimaryConstraint Primary Key(column(s));

Answer 2

There's really not much to it:
CREATE TABLE allocation (
staffID CHAR (4)
CONSTRAINT ALLOCATION_FK1
REFERENCES STAFF(STAFFID)
ON DELETE CASCADE,
projNo CHAR (4)
CONSTRAINT ALLOCATION_FK2
REFERENCES PROJECT(PROJNO)
ON DELETE CASCADE,
hours int (2),
CONSTRAINT PK_ALLOCATION
PRIMARY KEY (STAFFID, PROJNO)
USING INDEX
);
CREATE INDEX ALLOCATION_1
ON ALLOCATION (STAFFID);
CREATE INDEX ALLOCATION_2
ON ALLOCATION (PROJNO);
Here I've defined the foreign key constraints as part of the column definition, but there's no reason other than convenience to do it this way; you could, if you chose, make them out-of-line constraints. Because the primary key consists of multiple columns you have to make it an out-of-line constraint as shown here.
I've also defined indexes on both of the foreign keys. This is important - delete performance from the parent tables will suffer if you don't do this.
Best of luck.

Answer 3

You can set combination key for staffID CHAR (4) , projNo CHAR (4) for these two columns. and then add foreign key as usual.
CREATE TABLE [allocation](
[staffID] [char](4) NOT NULL,
[projNo] [char](4) NOT NULL,
[hours] [int] NULL,
CONSTRAINT [PK_allocation] PRIMARY KEY CLUSTERED
(
[staffID] ASC,
[projNo] ASC
))
Then set Foreign key with the help of alter command
ALTER TABLE [dbo].[allocation] WITH CHECK ADD CONSTRAINT [FK_allocation_project] FOREIGN KEY([projNo])
REFERENCES [dbo].[project] ([projNo])
GO
ALTER TABLE [dbo].[allocation] CHECK CONSTRAINT [FK_allocation_project]
GO
ALTER TABLE [dbo].[allocation] WITH CHECK ADD CONSTRAINT [FK_allocation_staff] FOREIGN KEY([staffID])
REFERENCES [dbo].[staff] ([staffID])
GO
ALTER TABLE [dbo].[allocation] CHECK CONSTRAINT [FK_allocation_staff]
GO