Author: Written by Herouth Maoz (<herouth@oumail.openu.ac.il>). This originally appeared on the User's Mailing List on 1998-03-02 in response to the question: "What is the difference between PRIMARY KEY and UNIQUE constraints?".
Subject: Re: [QUESTIONS] PRIMARY KEY | UNIQUE
What's the difference between:
PRIMARY KEY(fields,...) and
UNIQUE (fields,...)
- Is this an alias?
- If PRIMARY KEY is already unique, then why
is there another kind of key named UNIQUE?
A primary key is the field(s) used to identify a specific row. For example, Social Security numbers identifying a person.
A simply UNIQUE combination of fields has nothing to do with identifying the row. It's simply an integrity constraint. For example, I have collections of links. Each collection is identified by a unique number, which is the primary key. This key is used in relations.
However, my application requires that each collection will also have a unique name. Why? So that a human being who wants to modify a collection will be able to identify it. It's much harder to know, if you have two collections named "Life Science", the the one tagged 24433 is the one you need, and the one tagged 29882 is not.
So, the user selects the collection by its name. We therefore make sure, within the database, that names are unique. However, no other table in the database relates to the collections table by the collection Name. That would be very inefficient.
Moreover, despite being unique, the collection name does not actually define the collection! For example, if somebody decided to change the name of the collection from "Life Science" to "Biology", it will still be the same collection, only with a different name. As long as the name is unique, that's OK.
So:
Primary key:
Is used for identifying the row and relating to it.
Is impossible (or hard) to update.
Should not allow NULLs.
Unique field(s):
Are used as an alternative access to the row.
Are updatable, so long as they are kept unique.
NULLs are acceptable.
As for why no non-unique keys are defined explicitly in standard SQL syntax? Well, you must understand that indices are implementation-dependent. SQL does not define the implementation, merely the relations between data in the database. Postgres does allow non-unique indices, but indices used to enforce SQL keys are always unique.
Thus, you may query a table by any combination of its columns, despite the fact that you don't have an index on these columns. The indexes are merely an implementation aid that each RDBMS offers you, in order to cause commonly used queries to be done more efficiently. Some RDBMS may give you additional measures, such as keeping a key stored in main memory. They will have a special command, for example
CREATE MEMSTORE ON table COLUMNS cols(This is not an existing command, just an example.)
In fact, when you create a primary key or a unique combination of fields, nowhere in the SQL specification does it say that an index is created, nor that the retrieval of data by the key is going to be more efficient than a sequential scan!
So, if you want to use a combination of fields that is not unique as a secondary key, you really don't have to specify anything - just start retrieving by that combination! However, if you want to make the retrieval efficient, you'll have to resort to the means your RDBMS provider gives you - be it an index, my imaginary MEMSTORE command, or an intelligent RDBMS that creates indices without your knowledge based on the fact that you have sent it many queries based on a specific combination of keys... (It learns from experience).