primary-key

From my reading, I understand what makes a good primary key, what a foreign key is and what a candidate key is. I've read in several different books and sources that: A foreign key must point to a candidate key (or primary) A foreign key almost always points to a primary key The authors of the sources always say something along the lines of, "while foreign keys can point at a candidate key (not primary) they seem to". Are there any examples of why you might choose a candidate key and not the primary key? Thank you Primary keys (PKs) have no role in relational theory. (Eg integrity or

I need to create a primary key scheme for a system that will need peer to peer replication. So I'm planning to combine a unique system ID and a sequential number in some way to come up with unique ID's. I want to make sure I'll never run out of ID's, so I'm thinking about using a varchar field, since I could always add another character if I start running out. But I've read that integers are better optimized for this. So I have some questions... 1) Are integers really better optimized? And if they are, how much of a performance difference is there between varchars and integers? I'm going to

I want to create a primary key(auto-increment) which is start with A001000001 . Here A001 would be constant and 000001 would be iterate. I want rows like this: How can I do it using SQL query? For SQL Server (you didn't clearly specify which RDBMS you're using), my suggestion would be: add a INT IDENTITY column to your table - and quite frankly, I would make that column the primary key add a computed column to your table that does this "prefixing" Something like: CREATE TABLE dbo.YourTable ( ID INT IDENTITY(1,1) PRIMARY KEY CLUSTERED, EmpID AS 'A001' + RIGHT('0000000' + CAST(ID AS VARCHAR(10))

I checked this SO post: What's the difference between primary key, unique key, and index in MySQL? and found the statement: Also note that columns defined as primary keys or unique keys are automatically indexed in MySQL. Based on this, I have two questions: Am I safe in assuming that there is no performance benefit to creating an index on a primary key itself because the primary key, by design, is an index? Perhaps the more important question: If you are doing the classic example people cite, doing SELECT based on lastName and firstName, and that table has a primary key that you SELECT by