Prompt me, what for need "primary key" in the database table?

Primary key is the other way to create a unique clustered index, just like
unique key is equal to create unique nonclustered index. There is no big
difference between primary/unique key contraint v.s direct create unique
clustered/nonclustered index statement.

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Gang He
Software Design Engineer
Microsoft SQL Server Storage Engine

This posting is provided "AS IS" with no warranties, and confers no rights.
"Alexandr" <alex (AT) trinity (DOT) ntu-kpi.kiev.ua> wrote

If a table does not have a primary key, then BY DEFINITION it is not a
table. You need to read a book on RDBMS foundations and the relational
model. .

Logical identification of an entity.

A key, by definition, is an attribute or set of attributes which can
uniquely identify an entity in the conceptual model. In the logical
representation, a column or set of columns which can uniquely identify a row
in a table is termed as a key. In practice, an entity may have many such
candidates for key ( hence called candidate key ), however for practical
purposes one out of those candidate keys has to be considered primal ( hence
called primary key ). The considerations should include Familiarity (
meaningful & familiar to the user ), Stability ( should not be altered
frequently ), Simplicity ( so that relational expressions can be effective )
and Irreducibility ( or Minimality which loosely suggests no subset of the
key should uniquely identify a row in the table )

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Anith

Keys are objects in rows, not tables; each row requires one PK.

It is the unique identifier, without which a row cannot be positively
identified.

It allows a row to be connected ('related') to other rows, when the other
row has a field with the same data type and the same value. From there, you
can build relational database systems.


"Alexandr" <alex (AT) trinity (DOT) ntu-kpi.kiev.ua> wrote