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Auto Generated Sureget Key In Sql Server

 

Jun 24, 2012 A surrogate key is an auto generated value, usually integer, in the dimension table. It is made the primary key of the table and is used to join a dimension to a fact table. Among other benefits, surrogate keys allow you to maintain history in a dimension table. Sep 13, 2009  13 September 2009. Identity over-use: Surrogate vs Composite keys in SQL Server. The SQL Server ‘Identity’ column is a handy way of ensuring a unique primary key, but I have noticed a tendency for some database designs to over-use it. When designing a table, we often use the surrogate primary key whose values are sequential integers generated automatically by the database system. This primary key column is known as an identity or auto increment column. When a new row is inserted into the auto-increment column, an auto-generated sequential integer is used for the insert.

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Recommendations and examples for using the IDENTITY property to create surrogate keys on tables in Synapse SQL pool.

What is a surrogate key

A surrogate key on a table is a column with a unique identifier for each row. The key is not generated from the table data. Data modelers like to create surrogate keys on their tables when they design data warehouse models. You can use the IDENTITY property to achieve this goal simply and effectively without affecting load performance.

Creating a table with an IDENTITY column

The IDENTITY property is designed to scale out across all the distributions in the Synapse SQL pool without affecting load performance. Therefore, the implementation of IDENTITY is oriented toward achieving these goals.

You can define a table as having the IDENTITY property when you first create the table by using syntax that is similar to the following statement:

You can then use INSERT.SELECT to populate the table.

This remainder of this section highlights the nuances of the implementation to help you understand them more fully.

Allocation of values

The IDENTITY property doesn't guarantee the order in which the surrogate values are allocated, which reflects the behavior of SQL Server and Azure SQL Database. However, in Synapse SQL pool, the absence of a guarantee is more pronounced.

The following example is an illustration:

In the preceding example, two rows landed in distribution 1. The first row has the surrogate value of 1 in column C1, and the second row has the surrogate value of 61. Both of these values were generated by the IDENTITY property. However, the allocation of the values is not contiguous. This behavior is by design.

Skewed data

The range of values for the data type are spread evenly across the distributions. If a distributed table suffers from skewed data, then the range of values available to the datatype can be exhausted prematurely. For example, if all the data ends up in a single distribution, then effectively the table has access to only one-sixtieth of the values of the data type. For this reason, the IDENTITY property is limited to INT and BIGINT data types only.

SELECT.INTO

When an existing IDENTITY column is selected into a new table, the new column inherits the IDENTITY property, unless one of the following conditions is true:

  • The SELECT statement contains a join.
  • Multiple SELECT statements are joined by using UNION.
  • The IDENTITY column is listed more than one time in the SELECT list.
  • The IDENTITY column is part of an expression.

If any one of these conditions is true, the column is created NOT NULL instead of inheriting the IDENTITY property.

CREATE TABLE AS SELECT

CREATE TABLE AS SELECT (CTAS) follows the same SQL Server behavior that's documented for SELECT.INTO. However, you can't specify an IDENTITY property in the column definition of the CREATE TABLE part of the statement. You also can't use the IDENTITY function in the SELECT part of the CTAS. To populate a table, you need to use CREATE TABLE to define the table followed by INSERT.SELECT to populate it.

Explicitly inserting values into an IDENTITY column

Synapse SQL pool supports SET IDENTITY_INSERT <your table> ON OFF syntax. You can use this syntax to explicitly insert values into the IDENTITY column.

Many data modelers like to use predefined negative values for certain rows in their dimensions. An example is the -1 or 'unknown member' row.

The next script shows how to explicitly add this row by using SET IDENTITY_INSERT:

Loading data

The presence of the IDENTITY property has some implications to yourt be used:

Jan 20, 2020  I was recently doing some proof-of-concept work that required performing encryption using keys generated from AWS Key Management Service (KMS). I could find plenty of examples using symmetric encryption, but couldn’t find an end-to-end guide that showed how to generate keys from AWS and then use them to encrypt and decrypt data. To generate a data key, specify the symmetric CMK that will be used to encrypt the data key. You cannot use an asymmetric CMK to generate data keys. To get the type of your CMK, use the DescribeKey operation. You must also specify the length of the data key. Use either the KeySpec or NumberOfBytes parameters (but not both). May 01, 2019  kmsgeneratedatakey: Generate a data encryption key for envelope encryption In AWR.KMS: A Simple Client to the 'AWS' Key Management Service. Description Usage Arguments Value References. Generate a data encryption key for envelope encryption. Generate data key from kms. To generate a data key, you must specify the symmetric customer master key (CMK) that is used to encrypt the data key. You cannot use an asymmetric CMK to generate a data key. To get the type of your CMK, use the KeySpec field in the DescribeKey response. Generates a unique data key. This operation returns a plaintext copy of the data key and a copy that is encrypted under a customer master key (CMK) that you specify. You can use the plaintext key to encrypt your data outside of KMS and store the encrypted data key with the encrypted data.

  • When the column data type is not INT or BIGINT
  • When the column is also the distribution key
  • When the table is an external table

The following related functions are not supported in Synapse SQL pool:

Common tasks

This section provides some sample code you can use to perform common tasks when you work with IDENTITY columns.

Column C1 is the IDENTITY in all the following tasks.

Auto

Find the highest allocated value for a table

Use the MAX() function to determine the highest value allocated for a distributed table:

Find the seed and increment for the IDENTITY property

You can use the catalog views to discover the identity increment and seed configuration values for a table by using the following query:

Database Surrogate Key Definition

Next steps