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Summary: in this tutorial, you will learn how to use MySQL primary key constraint to create the primary key for a table.
A primary key in SQL Table helps to identify each row/record in a table uniquely. Here is the characteristic of Primary Key in SQL Table Primary keys must contain unique values; primary key column cannot have NULL values; Table can have only one primary key (can be composite with multiple fields). Summary: in this tutorial, you will learn how to use the SQL Server PRIMARY KEY constraint to create a primary key for a table. Introduction to SQL Server PRIMARY KEY constraint. A primary key is a column or a group of columns that uniquely identifies each row in a table. You create a primary key for a table by using the PRIMARY KEY constraint. If the primary key consists of only one column, you can define use PRIMARY KEY constraint as a column constraint.
Introduction to MySQL primary key
Ms project. A primary key is a column or a set of columns that uniquely identifies each row in the table. The primary key follows these rules:
- A primary key must contain unique values. If the primary key consists of multiple columns, the combination of values in these columns must be unique.
- A primary key column cannot have
NULL
values. Any attempt to insert or updateNULL
to primary key columns will result in an error. Note that MySQL implicitly adds aNOT NULL
constraint to primary key columns. - A table can have one an only one primary key.
Because MySQL works faster with integers, the data type of the primary key column should be the integer e.g.,
INT,
BIGINT
. And you should ensure sure that value ranges of the integer type for the primary key are sufficient for storing all possible rows that the table may have.A primary key column often has the
AUTO_INCREMENT
attribute that automatically generates a sequential integer whenever you insert a new row into the table.When you define a primary key for a table, MySQL automatically creates an index called
PRIMARY
.MySQL PRIMARY KEY
examples
The
PRIMARY KEY
constraint allows you to define a primary key of a table when you create or alter table.1) Define a PRIMARY KEY
constraint in CREATE TABLE
![Sql Sql](/uploads/1/2/6/0/126087002/554821534.jpg)
Typically, you define the primary key for a table in the
CREATE TABLE
statement.If the primary key has one column, you can use the
PRIMARY KEY
constraint as a column constraint: Bitcoin private and public key generator.When the primary key has more than one column, you must use the
PRIMARY KEY
constraint as a table constraint.In this syntax, you separate columns in the
column_list
by commas (,).The
PRIMARY KEY
table constraint can be used when the primary key has one column:The following example creates a table named
users
whose primary key is the user_id
column:This statement creates the
roles
table that has the PRIMARY KEY
constraint as the table constraint:In case the primary key consists of multiple columns, you must specify them at the end of the
CREATE TABLE
statement. You put a comma-separated list of primary key columns inside parentheses followed the PRIMARY KEY
keywords.The following example creates the
user_roles
table whose primary key consists of two columns: user_id
and role_id
. It defines the PRIMARY KEY
constraint as the table constraint:Note that the statement also created two foreign key constraints.
2) Define PRIMARY KEY
constraints using ALTER TABLE
If a table, for some reasons, does not have a primary key, you can use the
ALTER TABLE
statement to add a primary key to the table as follows:The following example adds the
id
column to the primary key.First, create the
pkdemos
table without a primary key.Second, add a primary key to the
pkdemos
table using the ALTER TABLE
statement:If you add a primary key to a table that already has data. The data in the column(s), which will be included in the primary key, must be unique and not NULL.
PRIMARY KEY
vs. UNIQUE KEY
vs. KEY
KEY
is the synonym for INDEX
. You use the KEY
when you want to create an index for a column or a set of columns that is not the part of a primary key or unique key.A
UNIQUE
index ensures that values in a column must be unique. Unlike the PRIMARY
index, MySQL allows NULL
values in the UNIQUE
index. In addition, a table can have multiple UNIQUE
indexes.Suppose that
email
and username
of users in the users
table must be unique. To enforce thes rules, you can define UNIQUE
indexes for the email
and username
columns as the following statement:Add a
UNIQUE
index for the username
column:Sql Server Generate Primary Key
Add a
UNIQUE
index for the email
column:Create Table Sql Primary Key
In this tutorial, you have learned how to create a primary key for a new table or add a primary key to an existing table.
Sql Generate Primary Key Value Chart
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