Surrogate keys are unique. Because surrogate keys are system-generated, it is impossible for the system to create and store a duplicate value. Surrogate keys apply uniform rules to all records. The surrogate key value is the result of a program, which creates the system-generated value. How to create a surrogate key in SQL Server Most database management systems have features to allow us to create a surrogate key column in our table. Surrogate keys are mostly maintained by the system and used to make several aspects of implementation easier. Can we implement surrogate key and unique index for hive table? I want to auto-increment the value of one of my column in hive table. And make another column a unique key which does not allow duplicate insertion into the hive table. Using IDENTITY to create surrogate keys in Synapse SQL pool.; 5 minutes to read; In this article. 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.
-->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
![Udf To Generate Surrogate Keys Udf To Generate Surrogate Keys](/uploads/1/2/6/0/126093595/702566771.png)
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.Udf To Generate Surrogate Keys 2016
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:
- 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:
![Keys Keys](/uploads/1/2/6/0/126093595/710235337.jpg)
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.
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
Udf To Generate Surrogate Keys Free
You can use the catalog views to discover the identity increment and seed configuration values for a table by using the following query:
Next steps
Goal
Fill in a data warehouse dimension table with data which comes from different source systems and assign a unique record identifier (surrogate key) to each record.
Scenario overview and details
To illustrate this example, we will use two made up sources of information to provide data about customers dimension. Each extract contains customer records with a business key (natural key) assigned to it.
In order to isolate the data warehouse from source systems, we will introduce a technical surrogate key instead of re-using the source system's natural (business) key.
A unique and common surrogate key is a one-field numeric key which is shorter, easier to maintain and understand, and independent from changes in source system than using a business key. Also, if a surrogate key generation process is implemented correctly, adding a new source system to the data warehouse processing will not require major efforts.
Surrogate key generation mechanism may vary depending on the requirements, however the inputs and outputs usually fit into the design shown below:
Inputs:
- an input respresented by an extract from the source system
- datawarehouse table reference for identifying the existing records
- maximum key lookup
Outputs:
- output table or file with newly assigned surrogate keys
- new maximum key
- updated reference table with new records
In order to isolate the data warehouse from source systems, we will introduce a technical surrogate key instead of re-using the source system's natural (business) key.
A unique and common surrogate key is a one-field numeric key which is shorter, easier to maintain and understand, and independent from changes in source system than using a business key. Also, if a surrogate key generation process is implemented correctly, adding a new source system to the data warehouse processing will not require major efforts.
Surrogate key generation mechanism may vary depending on the requirements, however the inputs and outputs usually fit into the design shown below:
Inputs:
- an input respresented by an extract from the source system
- datawarehouse table reference for identifying the existing records
- maximum key lookup
Outputs:
- output table or file with newly assigned surrogate keys
- new maximum key
- updated reference table with new records
Proposed solution
Assumptions:
- The surrogate key field for our made up example is WH_CUST_NO.
- To make the example clearer, we will use SCD 1 to handle changing dimensions. This means that new records overwrite the existing data.
The ETL process implementation requires several inputs and outputs.
Input data:
- customers_extract.csv - first source system extract
- customers2.txt - second source system extract
- CUST_REF - a lookup table which contains mapping between natural keys and surrogate keys
- MAX_KEY - a sequence number which represents last key assignment
Output data:
- D_CUSTOMER - table with new records and correctly associated surrogate keys
- CUST_REF - new mappings added
- MAX_KEY sequence increased
The design of an ETL process for generating surrogate keys will be as follows:
- The surrogate key field for our made up example is WH_CUST_NO.
- To make the example clearer, we will use SCD 1 to handle changing dimensions. This means that new records overwrite the existing data.
The ETL process implementation requires several inputs and outputs.
Input data:
- customers_extract.csv - first source system extract
- customers2.txt - second source system extract
- CUST_REF - a lookup table which contains mapping between natural keys and surrogate keys
- MAX_KEY - a sequence number which represents last key assignment
Output data:
- D_CUSTOMER - table with new records and correctly associated surrogate keys
- CUST_REF - new mappings added
- MAX_KEY sequence increased
The design of an ETL process for generating surrogate keys will be as follows:
- PROD_REF table
- max_key sequence
- populate a new surrogate key and assign it to the record. The new key will be populated by incrementing the old maximum key by 1.
- insert a new record into the products table
- insert a new record into the mapping table (which stores business and surrogate keys mapping)
- update the new maximum key
Sample Implementations
Generation of surrogate key implementation in various ETL environments:PDI surrogate key - surrogate key generation example implemented in Pentaho Data Integration