What's the point of a candidate key?

Question

I\'m fairly new to database management and this question never seems to be answered in more than one sentence. All other SO answers say \"A candidate key is a minimal super

Answer 1

Candidate keys usually refer to those columns which could potentially be selected as the natural primary key. However, natural primary keys are also often a bad idea because they are unique but not unchanging (think of the havoc of changing millions of child rows because a company name changed) or because they are less efficient in joins than surrogate keys. Further, in real life, many potential candidate keys are not nearly stable enough for a true PK and are not as unique as we think. Emails, for instance, can be reused after an account is closed.

You add other indexes to specify uniqueness in order to maintain data integrity. For instance in your example, you are using a surrogate key to ensure uniqueness of a record. But this does not ensure that the person/phone combination is entered only once. So you would want to create a unique index for any possible candidate key (and this can consist of one or more fields) in the data if you are using a surrogate key. This ensures that the items which need to be unique are and allows you to get the performance benefits of a surrogate key.

Further sometimes there are mulitple fields or combinations of fields which should be unique. For instance suppose you have a table storing organization structure. Each organizational entity should be unique, so should each organization/person combination (assuming that there is no job sharing). By placing unique indexes on these fields, you are able to prevent bad data from being inserted into the table.