Joe Celko's Thinking in Sets: Auxiliary, Temporal, and Virtual Tables in SQL

10.1. Repeated Groups

SQL does not require that a table have unique constraints, a primary key, or anything else that would ensure data integrity. In short, you can use a table pretty much like a file if you wish. Is this a bad thing?

Well, mostly yes and a little no. You should never have such a beast in your final schema, but one common programming trick is to use a table without any constraints as a staging area. You load data from an external source into one of these pseudo-tables, scrub it, and pass it along to a real table in the actual schema. The trouble is that a lot of the time, the pseudo-table is denormalized as well as full of bad data. You can do some normalization from the staging table into another set of scrubbing tables, but ...