The first is PRIMARY KEY, which as the name suggests, forces the specified column to behave as a completely unique index for the table, allowing for rapid searching and queries. While SQL Server only allows one PRIMARY KEY constraint assigned to a single table, that PRIMARY KEY can be defined for more than one column. Key types and values. While EF Core supports using properties of any primitive type as the primary key, including string, Guid, byte and others, not all databases support all types as keys. In some cases the key values can be converted to a supported type automatically, otherwise the conversion should be specified manually.
-->You can define a primary key in SQL Server by using SQL Server Management Studio or Transact-SQL. Creating a primary key automatically creates a corresponding unique clustered index, or a nonclustered index if specified as such.
Before You Begin
Limitations and Restrictions
Union Auto Generate Primary Key Data
- A table can contain only one PRIMARY KEY constraint.
- All columns defined within a PRIMARY KEY constraint must be defined as NOT NULL. If nullability is not specified, all columns participating in a PRIMARY KEY constraint have their nullability set to NOT NULL.
Security
Permissions
Creating a new table with a primary key requires CREATE TABLE permission in the database and ALTER permission on the schema in which the table is being created.
Creating a primary key in an existing table requires ALTER permission on the table.
Using SQL Server Management Studio
To create a primary key
- In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design.
- In Table Designer, click the row selector for the database column you want to define as the primary key. If you want to select multiple columns, hold down the CTRL key while you click the row selectors for the other columns.
- Right-click the row selector for the column and select Set Primary Key.
Union Auto Generate Primary Key Id
Caution
If you want to redefine the primary key, any relationships to the existing primary key must be deleted before the new primary key can be created. A message will warn you that existing relationships will be automatically deleted as part of this process.
A primary key column is identified by a primary key symbol in its row selector.
If a primary key consists of more than one column, duplicate values are allowed in one column, but each combination of values from all the columns in the primary key must be unique.
If you define a compound key, the order of columns in the primary key matches the order of columns as shown in the table. However, you can change the order of columns after the primary key is created. For more information, see Modify Primary Keys.
Using Transact-SQL
To create a primary key in an existing table
The following example creates a primary key on the column
TransactionID
in the AdventureWorks database.![Generate Generate](/uploads/1/2/6/4/126450921/425252082.png)
To create a primary key in a new table
The following example creates a table and defines a primary key on the column
TransactionID
in the AdventureWorks database.To create a primary key with clustered index in a new table
The following example creates a table and defines a primary key on the column
CustomerID
and a clustered index on TransactionID
in the AdventureWorks database.See Also
The
AUTO_INCREMENT
attribute can be used to generate a unique identity for new rows: Which returns:
No value was specified for the
AUTO_INCREMENT
column, so MySQL assigned sequence numbers automatically. You can also explicitly assign 0 to the column to generate sequence numbers, unless the NO_AUTO_VALUE_ON_ZERO
SQL mode is enabled. For example: If the column is declared
NOT NULL
, it is also possible to assign NULL
to the column to generate sequence numbers. For example: When you insert any other value into an
AUTO_INCREMENT
column, the column is set to that value and the sequence is reset so that the next automatically generated value follows sequentially from the largest column value. For example: Updating an existing
AUTO_INCREMENT
column value in an InnoDB
table does not reset the AUTO_INCREMENT
sequence as it does for MyISAM
and NDB
tables. You can retrieve the most recent automatically generated
AUTO_INCREMENT
value with the LAST_INSERT_ID()
SQL function or the mysql_insert_id()
C API function. These functions are connection-specific, so their return values are not affected by another connection which is also performing inserts. Use the smallest integer data type for the
AUTO_INCREMENT
column that is large enough to hold the maximum sequence value you will need. When the column reaches the upper limit of the data type, the next attempt to generate a sequence number fails. Use the UNSIGNED
attribute if possible to allow a greater range. For example, if you use TINYINT
, the maximum permissible sequence number is 127. For TINYINT UNSIGNED
, the maximum is 255. See Section 11.1.2, “Integer Types (Exact Value) - INTEGER, INT, SMALLINT, TINYINT, MEDIUMINT, BIGINT” for the ranges of all the integer types. For a multiple-row insert,
LAST_INSERT_ID()
and mysql_insert_id()
actually return the AUTO_INCREMENT
key from the first of the inserted rows. This enables multiple-row inserts to be reproduced correctly on other servers in a replication setup. To start with an
AUTO_INCREMENT
value other than 1, set that value with CREATE TABLE
or ALTER TABLE
, like this: For information about
AUTO_INCREMENT
usage specific to InnoDB
, see Section 14.6.1.6, “AUTO_INCREMENT Handling in InnoDB”. - For
MyISAM
tables, you can specifyAUTO_INCREMENT
on a secondary column in a multiple-column index. In this case, the generated value for theAUTO_INCREMENT
column is calculated asMAX(
. This is useful when you want to put data into ordered groups.auto_increment_column
) + 1 WHERE prefix=given-prefix
Which returns:In this case (when theAUTO_INCREMENT
column is part of a multiple-column index),AUTO_INCREMENT
values are reused if you delete the row with the biggestAUTO_INCREMENT
value in any group. This happens even forMyISAM
tables, for whichAUTO_INCREMENT
values normally are not reused. - If the
AUTO_INCREMENT
column is part of multiple indexes, MySQL generates sequence values using the index that begins with theAUTO_INCREMENT
column, if there is one. For example, if theanimals
table contained indexesPRIMARY KEY (grp, id)
andINDEX (id)
, MySQL would ignore thePRIMARY KEY
for generating sequence values. As a result, the table would contain a single sequence, not a sequence pergrp
value.
More information about
AUTO_INCREMENT
is available here: - How to assign the
AUTO_INCREMENT
attribute to a column: Section 13.1.17, “CREATE TABLE Statement”, and Section 13.1.7, “ALTER TABLE Statement”. - How
AUTO_INCREMENT
behaves depending on theNO_AUTO_VALUE_ON_ZERO
SQL mode: Section 5.1.10, “Server SQL Modes”. - How to use the
LAST_INSERT_ID()
function to find the row that contains the most recentAUTO_INCREMENT
value: Section 12.15, “Information Functions”. - Setting the
AUTO_INCREMENT
value to be used: Section 5.1.7, “Server System Variables”. AUTO_INCREMENT
and replication: Section 17.4.1.1, “Replication and AUTO_INCREMENT”.- Server-system variables related to
AUTO_INCREMENT
(auto_increment_increment
andauto_increment_offset
) that can be used for replication: Section 5.1.7, “Server System Variables”.