Editing Primary key (section)

==Defining primary keys in SQL==
Primary keys are defined in the [[SQL#Standardization history|ISO SQL Standard]], through the PRIMARY KEY constraint. The syntax to add such a constraint to an existing table is defined in [[SQL:2003]] like this:

<syntaxhighlight lang="sql">
ALTER TABLE <table identifier> 
    ADD [ CONSTRAINT <constraint identifier> ] 
    PRIMARY KEY ( <column name> [ {, <column name> }... ] )
</syntaxhighlight>

The primary key can also be specified directly during table creation. In the SQL Standard, primary keys may consist of one or multiple columns. Each column participating in the primary key is implicitly                                                                          defined as NOT NULL. Note that some [[RDBMS]] require explicitly marking primary key columns as <code>NOT NULL</code>.{{citation needed|date=January 2013}}

<syntaxhighlight lang="sql">
CREATE TABLE table_name (
   
   ...
)
</syntaxhighlight>

If the primary key consists only of a single column, the column can be marked as such using the following syntax:

<syntaxhighlight lang="sql">
CREATE TABLE table_name (
   id_col  INT  PRIMARY KEY,
   col2    CHARACTER VARYING(20),
   ...
)
</syntaxhighlight>