SQL - When creating a table, how can I make it so a column is required when another column meets a certain requirement? - sql

Question

I am using Microsoft SQL Server Management Studio. So this is what I have right now:
CREATE TABLE Product
(
product_id INT NOT NULL PRIMARY KEY IDENTITY,
product_code CHAR(4) NOT NULL, --For a book use 'BOOK'
product_name VARCHAR(40) NOT NULL,
product_desc VARCHAR(5000),
book_author INT,
book_publisher INT,
product_price SMALLMONEY NOT NULL CHECK (product_price >= 0),
FOREIGN KEY (book_author) REFERENCES Author
);
So I would like to make it so that book_author and book_publisher cannot be null if product_code == 'BOOK'.
Is this possible and how?

Answer 1

This is how I would do it:
CREATE TABLE Product (
product_id INT NOT NULL PRIMARY KEY IDENTITY
, product_code CHAR(4) NOT NULL
, product_name VARCHAR(40) NOT NULL
, product_desc VARCHAR(5000)
, book_author INT
, book_publisher INT
, product_price SMALLMONEY NOT NULL CHECK (product_price >= 0)
, CONSTRAINT CHK_author CHECK (
CASE
WHEN product_code = 'BOOK' AND (book_author IS NULL OR book_publisher IS NULL) THEN 0
ELSE 1
END = 1
)
);
CHK_author constraint will check if your product_code is BOOK, and if it is, then it will check whether book_author OR book_publisher are NULL values. If one of them is - it will restrict statement.
Here's a SQL Fiddle

Answer 2

CREATE TABLE [dbo].[Product](
[product_id] [int] IDENTITY(1,1) NOT NULL,
[product_code] [char](4) NOT NULL,
[product_name] [varchar](40) NOT NULL,
[product_desc] [varchar](5000) NULL,
[book_author] [int] NULL,
[book_publisher] [int] NULL,
[product_price] [smallmoney] NOT NULL,
PRIMARY KEY CLUSTERED ( [product_id] ASC )
)
Go
ALTER TABLE [dbo].[Product] WITH CHECK ADD CONSTRAINT [CK_Product] CHECK (
(
( [product_code] <> 'BOOK' )
OR
(
( [book_author] IS NOT NULL )
AND ( [book_publisher] IS NOT NULL )
)
)
AND
( [product_price] >= 0 )
)
Go
ALTER TABLE [dbo].[Product] CHECK CONSTRAINT [CK_Product]
Go
One constraint pair row.