This article brings you relevant knowledge about SQL server, which mainly introduces SQL Server primary key constraints (PRIMARY KEY). The primary key is a column or a group that uniquely identifies each row in the table. Column, the article expands on the topic in detail. Let’s take a look at it together. I hope it will be helpful to everyone.
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Primary key Is a column or set of columns that uniquely identifies each row in a table. You can use primary key constraints to create a primary key for a table.
If the primary key contains only one column, you can use PRIMARY KEY
constraints as column constraints:
CREATE TABLE table_name ( pk_column data_type PRIMARY KEY, ... );
If the primary key has two or more columns, you must use the primary key constraint Table constraints:
CREATE TABLE table_name ( pk_column_1 data_type, pk_column_2 data type, ... PRIMARY KEY (pk_column_1, pk_column_2) );
Each table can only contain one primary key, and one primary key can contain multiple columns, that is, the combination of multiple columns cannot be repeated. All columns participating in the primary key must be defined as NOT NULL
. If NOT NULL
constraints are not specified for all primary key columns, SQL Server automatically sets non-null constraints for these columns.
The following example creates a table with a primary key consisting of one column:
CREATE TABLE dbo.activities ( activity_id INT PRIMARY KEY IDENTITY,--主键 activity_name VARCHAR (255) NOT NULL, activity_date DATE NOT NULL );
In table## In #dbo.activities, the
activity_id column is the primary key column, which means that the value of this column cannot be repeated.
IDENTITY attribute is used for
activity_idColumns automatically generate unique integer values.
The following creates a new table with two columns that form the foreign key:
CREATE TABLE dbo.participants( activity_id int, customer_id int, PRIMARY KEY(activity_id, customer_id) );
activity_id or
customer_id Values in columns can be repeated, but each combination of values in the two columns must be unique.
ALTER TABLE statement to add a primary key to the table. For example:
First create a table without a primary key column:
CREATE TABLE dbo.events( event_id INT NOT NULL, event_name VARCHAR(255), start_date DATE NOT NULL, duration DEC(5,2) );
event_id column a primary key:
ALTER TABLE sales.events ADD PRIMARY KEY(event_id);
Note that if theRecommended study: "sales.events
table already has data, before promoting the
event_idcolumn to the primary key, you must ensure that the value in
event_idis not Repeated.
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