Sql Server Sequence Generated Primary Key On Multiple Columns
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Summary: in this tutorial, you will learn how to use Oracle PRIMARY KEY constraint to manage the primary key of a table. Introduction to the primary key. A primary key is a column of a combination of columns in a table that uniquely identifies a row in the table. SQL PRIMARY KEY Constraint. The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields). I've also include a sample output after each query for reference. One other item to note is with the foreign key section. We need to remove the referenced primary key index in order to make the collation update to that column as well so that the section includes two queries. One to identify the foreign key and the other for the primary key. As discussed in the documentation, a sequence in Oracle prevents duplicate values from being created simultaneously because multiple users are effectively forced to “take turns” before each sequential item is generated. For the purposes of creating a unique primary key for a new table, first we must CREATE the table we’ll be using. Apr 24, 2007 For example, assigning each new Customer added to your table a unique 'CustomerNumber'. There are effectively two ways to do using the built-in features that T-SQL provides: Identity Columns - An identity is a common 'auto generated' primary key to use in a SQL Server database these days. An identity is simply an integer value that 'auto. What is surrogate key generation.
A sequence is a user-defined schema-bound object that generates a sequence of numeric values according to the specification with which the sequence was created. The sequence of numeric values is generated in an ascending or descending order at a defined interval and may cycle (repeat) as requested. Sequences, unlike identity columns, are not associated with tables. An application refers to a sequence object to receive its next value. The relationship between sequences and tables is controlled by the application. User applications can reference a sequence object and coordinate the values keys across multiple rows and tables.
A sequence is created independently of the tables by using the CREATE SEQUENCE statement. Options enable you to control the increment, maximum and minimum values, starting point, automatic restarting capability, and caching to improve performance. For information about the options, see CREATE SEQUENCE.
Unlike identity column values, which are generated when rows are inserted, an application can obtain the next sequence number before inserting the row by calling the NEXT VALUE FOR function. The sequence number is allocated when NEXT VALUE FOR is called even if the number is never inserted into a table. The NEXT VALUE FOR function can be used as the default value for a column in a table definition. Use sp_sequence_get_range to get a range of multiple sequence numbers at once.
A sequence can be defined as any integer data type. If the data type is not specified, a sequence defaults to bigint.
Using Sequences
Use sequences instead of identity columns in the following scenarios:
The application requires a number before the insert into the table is made.
The application requires sharing a single series of numbers between multiple tables or multiple columns within a table.
The application must restart the number series when a specified number is reached. For example, after assigning values 1 through 10, the application starts assigning values 1 through 10 again.
The application requires sequence values to be sorted by another field. The NEXT VALUE FOR function can apply the OVER clause to the function call. The OVER clause guarantees that the values returned are generated in the order of the OVER clause's ORDER BY clause.
An application requires multiple numbers to be assigned at the same time. For example, an application needs to reserve five sequential numbers. Requesting identity values could result in gaps in the series if other processes were simultaneously issued numbers. Calling sp_sequence_get_range can retrieve several numbers in the sequence at once.
skype para mac download espanol You need to change the specification of the sequence, such as the increment value.
Limitations
Unlike identity columns, whose values cannot be changed, sequence values are not automatically protected after insertion into the table. To prevent sequence values from being changed, use an update trigger on the table to roll back changes.
Uniqueness is not automatically enforced for sequence values. The ability to reuse sequence values is by design. If sequence values in a table are required to be unique, create a unique index on the column. If sequence values in a table are required to be unique throughout a group of tables, create triggers to prevent duplicates caused by update statements or sequence number cycling.
The sequence object generates numbers according to its definition, but the sequence object does not control how the numbers are used. Sequence numbers inserted into a table can have gaps when a transaction is rolled back, when a sequence object is shared by multiple tables, or when sequence numbers are allocated without using them in tables. When created with the CACHE option, an unexpected shutdown, such as a power failure, can lose the sequence numbers in the cache.
If there are multiple instances of the NEXT VALUE FOR function specifying the same sequence generator within a single Transact-SQL statement, all those instances return the same value for a given row processed by that Transact-SQL statement. This behavior is consistent with the ANSI standard.
Sequence numbers are generated outside the scope of the current transaction. They are consumed whether the transaction using the sequence number is committed or rolled back. Duplicate validation only occurs once a record is fully populated. This can result in some cases where the same number is used for more than one record during creation, but then gets identified as a duplicate. If this occurs and other autonumber values have been applied to subsequent records, this can result in a gap between autonumber values.
Typical Use
To create an integer sequence number that increments by 1 from -2,147,483,648 to 2,147,483,647, use the following statement.
To create an integer sequence number similar to an identity column that increments by 1 from 1 to 2,147,483,647, use the following statement.
Managing Sequences
For information about sequences, query sys.sequences.
Examples
Sql Server Two Primary Keys
There are additional examples in the topics CREATE SEQUENCE (Transact-SQL), NEXT VALUE FOR (Transact-SQL), and sp_sequence_get_range.
A. Using a sequence number in a single table
The following example creates a schema named Test, a table named Orders, and a sequence named CountBy1, and then inserts rows into the table using the NEXT VALUE FOR function.
Here is the result set.
OrderID Name Qty
1 Tire 2
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2 Seat 1
3 Brake 1
B. Calling NEXT VALUE FOR before inserting a row
Using the Orders table created in example A, the following example declares a variable named @nextID, and then uses the NEXT VALUE FOR function to set the variable to the next available sequence number. The application is presumed to do some processing of the order, such as providing the customer with the OrderID number of their potential order, and then validates the order. No matter how long this processing might take, or how many other orders are added during the process, the original number is preserved for use by this connection. Finally, the INSERT statement adds the order to the Orders table.
C. Using a sequence number in multiple tables
This example assumes that a production-line monitoring process receives notification of events that occur throughout the workshop. Each event receives a unique and monotonically increasing EventID number. All events use the same EventID sequence number so that reports that combine all events can uniquely identify each event. However the event data is stored in three different tables, depending on the type of event. The code example creates a schema named Audit, a sequence named EventCounter, and three tables which each use the EventCounter sequence as a default value. Then the example adds rows to the three tables and queries the results.
Here is the result set.
EventID EventTime Description
1 2009-11-02 15:00:51.157 Start
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2 2009-11-02 15:00:51.160 Start
3 2009-11-02 15:00:51.167 Clean room temperature 18 degrees C.
4 2009-11-02 15:00:51.167 Spin rate threshold exceeded.
5 2009-11-02 15:00:51.173 Feeder jam
6 2009-11-02 15:00:51.177 Stop
7 2009-11-02 15:00:51.180 Central feed in bypass mode.
D. Generating repeating sequence numbers in a result set
The following example demonstrates two features of sequence numbers: cycling, and using NEXT VALUE FOR in a select statement.
E. Generating sequence numbers for a result set by using the OVER clause
The following example uses the OVER clause to sort the result set by Name before it adds the sequence number column.
Sql Server Sequence Generated Primary Key On Multiple Columns List
F. Resetting the sequence number
Example E consumed the first 79 of the Samples.IDLabel sequence numbers. (Your version of AdventureWorks2012 may return a different number of results.) Execute the following to consume the next 79 sequence numbers (80 though 158).
Execute the following statement to restart the Samples.IDLabel sequence.
Execute the select statement again to verify that the Samples.IDLabel sequence restarted with number 1.
G. Changing a table from identity to sequence
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The following example creates a schema and table containing three rows for the example. Then the example adds a new column and drops the old column.
Sql Server Sequence Generated Primary Key On Multiple Columns List
Transact-SQL statements that use SELECT * will receive the new column as the last column instead of the first column. If this is not acceptable, then you must create an entirely new table, move the data to it, and then recreate the permissions on the new table.