NULL
values. Any attempt to insert or updateNULL
to primary key columns will result in an error. Note that MySQL implicitly adds a NOT NULL
constraint to primary key columns.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.AUTO_INCREMENT
attribute that automatically generates a sequential integer whenever you insert a new row into the table.PRIMARY
.PRIMARY KEY
examplesPRIMARY KEY
constraint allows you to define a primary key of a table when you create or alter table.PRIMARY KEY
constraint in CREATE TABLE
CREATE TABLE
Ogniem i mieczem filmweb. statement.PRIMARY KEY
constraint as a column constraint:PRIMARY KEY
constraint as a table constraint.column_list
by commas (,).PRIMARY KEY
table constraint can be used when the primary key has one column:users
whose primary key is the user_id
column:roles
table that has the PRIMARY KEY
constraint as the table constraint:CREATE TABLE
statement. You put a comma-separated list of primary key columns inside parentheses followed the PRIMARY KEY
keywords.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:PRIMARY KEY
constraints using ALTER TABLE
ALTER TABLE
statement to add a primary key to the table as follows:id
column to the primary key.pkdemos
table without a primary key.pkdemos
table using the ALTER TABLE
statement: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.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.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:UNIQUE
index for the username
column:UNIQUE
index for the email
column: