Sams Teach Yourself SQL in 21 Days, Fourth Edition

Q&A

Q1:	Can constraints cause problems when importing data into a database?
A1:	Yes, constraints are often violated when trying to import data, say from one database into another. You might want to disable constraints before the import and then enable the constraints after the import. Keep in mind, though, that if there is data in a table that violates a disabled constraint, the constraint cannot be enabled. You have to fix the data, remove duplicate records, or whatever is required first, and then enable the constraint.
Q2:	Can you have more than one column in a table that makes up a primary key?
A2:	Yes, multiple columns can be used to define the primary key for a table. The combination of the columns that define the primary key must be a unique ...