primary key is used to uniquely
identify each row in a table. It
can either be part of the actual
record itself , or it can be an
artificial field (one that has
nothing to do with the actual
record). A primary key can consist
of one or more fields on a table.
When multiple fields are used as a
primary key, they are called a
composite key.
Primary keys can be specified
either when the table is created
(using CREATE
TABLE) or by changing the
existing table structure (using ALTER
TABLE).
Below are examples for
specifying a primary key when
creating a table:
MySQL:
CREATE
TABLE Customer
(SID integer,
Last_Name varchar(30),
First_Name varchar(30),
PRIMARY KEY (SID));
Oracle:
CREATE
TABLE Customer
(SID integer PRIMARY KEY,
Last_Name varchar(30),
First_Name varchar(30));
SQL Server:
CREATE
TABLE Customer
(SID integer PRIMARY KEY,
Last_Name varchar(30),
First_Name varchar(30));
Below are examples for
specifying a primary key by
altering a table:
MySQL:
ALTER
TABLE Customer ADD PRIMARY KEY
(SID);
Oracle:
ALTER
TABLE Customer ADD PRIMARY KEY
(SID);
SQL Server:
ALTER
TABLE Customer ADD PRIMARY KEY
(SID);
Note: Before using the ALTER
TABLE command to add a primary
key, you'll need to make sure that
the field is defined as 'NOT NULL'
-- in other words, NULL cannot be
an accepted value for that field.
