primary key

primary key

   <database> A unique {identifier}, often an {integer}, that
   labels a certain row in a table of a {relational database}.

   When this value occurs in other tables as a reference to a
   particular row in the first table it is called a "foreign
   key".

   Some {RDBMS}es can generate a new unique identifier each time
   a new row is inserted, others merely allow a column to be
   constrained to contain unique values.

   A table may have multiple {candidate keys}, from which the
   primary key is chosen.  The primary key should be an arbitrary
   value, such as an {autoincrementing} integer.  This avoids
   dependence on uniqueness, permanence and format of existing
   columns with real-world meaning (e.g. a person's name) or
   other external identifier (e.g. social security number).

   There should be enough possible primary key values to cater
   for the current and expected number of rows, bearing in mind
   that a wider column will generally be slower to process.

   (2006-05-29)