Forcing Unique Values with UNIQUE
A unique constraint ensures that a column (or set of columns) contains no duplicate values. A unique constraint is similar to a primary-key constraint, except that a unique column can contain nulls and a table can have multiple unique columns. (For information about primary-key constraints, see “Specifying a Primary Key with PRIMARY KEY” earlier in this chapter.)
Suppose that I add the column isbn to the table titles to hold a book’s ISBN. An ISBN is a unique, standardized identification number that marks a book unmistakably. titles already has a primary key (title_id), so to ensure that each ISBN value is unique, I can define a unique constraint on the column isbn.
When you’re defining a unique constraint, ...
Get SQL: Visual QuickStart Guide now with the O’Reilly learning platform.
O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers.
