March 2001
Intermediate to advanced
640 pages
14h 9m
English
book
Sams Teach Yourself Transact-SQL in 21 Days, Second Edition
by Lowell Mauer, David Solomon, Bennett McEwan
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Defining Constraints
Constraints are defined during the CREATE TABLE or ALTER TABLE command. They are used to enforce data integrity and ensure that valid data is entered into the columns. Constraints can also ensure that integrity is maintained between tables. A column-level constraint applies to a single column, whereas a table-level constraint applies to multiple columns, even if it does not apply to every column. PRIMARY KEY, UNIQUE, FOREIGN KEY, DEFAULT, and CHECK are all examples of constraints. Both PRIMARY KEY and UNIQUE constraints are used to enforce uniqueness on columns. However, a UNIQUE constraint can allow NULLs.
Tip
If you need to know the constraints already on a table, you can use the stored procedure
sp_helpconstraint table_name ...Become an O’Reilly member and get unlimited access to this title plus top books and audiobooks from O’Reilly and nearly 200 top publishers, thousands of courses curated by job role, 150+ live events each month,
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.