Answer Posted / vinod singh kushwah
A 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);
| Is This Answer Correct ? | 6 Yes | 1 No |
Post New Answer View All Answers
What happens if you lost a data file?
What is oracle analytical function?
What is the Tune Query
What are transaction isolation levels supported by oracle?
what is insert all statement in sql
How to define an explicit cursor in oracle?
How to run the anonymous block again?
What privilege is needed for a user to create indexes in oracle?
IS it possible to built the oracle database without setting the kernal parameters?
How do we represent comments in oracle?
Why oracle is used?
Typically, where is the conventional directory structure chosen for Oracle binaries to reside?
How to write a query with an inner join in oracle?
What are the differences between date and timestamp in oracle?
Explain the use of indexes option in exp command.
