1. Data Warehouse Surrogate Key
2. Data Warehouse Surrogate Key Generation 2017
3. Surrogate Key
4. Data Warehouse Surrogate Key Generation Model
5. What Is Surrogate Key

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

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Application data is not used to derive surrogate key. Surrogate key is an internally generated key by the current system and is invisible to the user. As several objects are available in the database corresponding to surrogate, surrogate key can not be utilized as primary key. For example: A sequential number can be a surrogate key. Surrogate keys are keys that are maintained within the data warehouse instead of natural keys which are taken from source data systems. There are several reasons for the use of surrogate keys: Allows integrating data from multiple source systems, (i.e. In case two source systems use the same value in natural key field). Oct 14, 2017 Why Surrogate Keys are used in Data Warehouse aroundBI. Unsubscribe from aroundBI? Database Design 25 - Surrogate Key and Natural Key - Duration: 7:34.

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.

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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:

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

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

A surrogate key (or synthetic key, entity identifier, system-generated key, database sequence number, factless key, technical key, or arbitrary unique identifier^{[citation needed]}) in a database is a unique identifier for either an entity in the modeled world or an object in the database. The surrogate key is not derived from application data, unlike a natural (or business) key which is derived from application data.^[1]

Definition[edit]

There are at least two definitions of a surrogate:

Surrogate (1) – Hall, Owlett and Todd (1976)

A surrogate represents an entity in the outside world. The surrogate is internally generated by the system but is nevertheless visible to the user or application.^[2]

Surrogate (2) – Wieringa and De Jonge (1991)

A surrogate represents an object in the database itself. The surrogate is internally generated by the system and is invisible to the user or application.

The Surrogate (1) definition relates to a data model rather than a storage model and is used throughout this article. See Date (1998).

An important distinction between a surrogate and a primary key depends on whether the database is a current database or a temporal database. Since a current database stores only currently valid data, there is a one-to-one correspondence between a surrogate in the modeled world and the primary key of the database. In this case the surrogate may be used as a primary key, resulting in the term surrogate key. In a temporal database, however, there is a many-to-one relationship between primary keys and the surrogate. Since there may be several objects in the database corresponding to a single surrogate, we cannot use the surrogate as a primary key; another attribute is required, in addition to the surrogate, to uniquely identify each object.

Although Hall et al. (1976) say nothing about this, others^[specify] have argued that a surrogate should have the following characteristics:

• the value is unique system-wide, hence never reused
• the value is system generated
• the value is not manipulable by the user or application
• the value contains no semantic meaning
• the value is not visible to the user or application
• the value is not composed of several values from different domains.

Surrogates in practice[edit]

In a current database, the surrogate key can be the primary key, generated by the database management system and not derived from any application data in the database. The only significance of the surrogate key is to act as the primary key. It is also possible that the surrogate key exists in addition to the database-generated UUID (for example, an HR number for each employee other than the UUID of each employee).

A surrogate key is frequently a sequential number (e.g. a Sybase or SQL Server 'identity column', a PostgreSQL or Informixserial, an Oracle or SQL ServerSEQUENCE or a column defined with AUTO_INCREMENT in MySQL). Some databases provide UUID/GUID as a possible data type for surrogate keys (e.g. PostgreSQL UUID or SQL Server UNIQUEIDENTIFIER).

Having the key independent of all other columns insulates the database relationships from changes in data values or database design (making the database more agile) and guarantees uniqueness.

In a temporal database, it is necessary to distinguish between the surrogate key and the business key. Every row would have both a business key and a surrogate key. The surrogate key identifies one unique row in the database, the business key identifies one unique entity of the modeled world. One table row represents a slice of time holding all the entity's attributes for a defined timespan. Those slices depict the whole lifespan of one business entity. For example, a table EmployeeContracts may hold temporal information to keep track of contracted working hours. The business key for one contract will be identical (non-unique) in both rows however the surrogate key for each row is unique.

Some database designers use surrogate keys systematically regardless of the suitability of other candidate keys, while others will use a key already present in the data, if there is one.

Some of the alternate names ('system-generated key') describe the way of generating new surrogate values rather than the nature of the surrogate concept.

Approaches to generating surrogates include:

• Universally Unique Identifiers (UUIDs)
• Globally Unique Identifiers (GUIDs)
• Object Identifiers (OIDs)
• Sybase or SQL Server identity column IDENTITY OR IDENTITY(n,n)
• OracleSEQUENCE, or GENERATED AS IDENTITY (starting from version 12.1)^[3]
• SQL ServerSEQUENCE (starting from SQL Server 2012)^[4]
• PostgreSQL or IBM Informix serial
• MySQLAUTO_INCREMENT
• SQLiteAUTOINCREMENT
• AutoNumber data type in Microsoft Access
• AS IDENTITY GENERATED BY DEFAULT in IBM DB2
• Identity column (implemented in DDL) in Teradata
• Table Sequence when the sequence is calculated by a procedure and a sequence table with fields: id, sequenceName, sequenceValue and incrementValue

Advantages[edit]

Immutability[edit]

Surrogate keys do not change while the row exists. This has the following advantages:

• Applications cannot lose their reference to a row in the database (since the identifier never changes).
• The primary or natural key data can always be modified, even with databases that do not support cascading updates across related foreign keys.

Requirement changes[edit]

Attributes that uniquely identify an entity might change, which might invalidate the suitability of natural keys. Consider the following example:

An employee's network user name is chosen as a natural key. Upon merging with another company, new employees must be inserted. Some of the new network user names create conflicts because their user names were generated independently (when the companies were separate).

In these cases, generally a new attribute must be added to the natural key (for example, an original_company column).With a surrogate key, only the table that defines the surrogate key must be changed. With natural keys, all tables (and possibly other, related software) that use the natural key will have to change.

Some problem domains do not clearly identify a suitable natural key. Surrogate keys avoid choosing a natural key that might be incorrect.

Performance[edit]

Data Warehouse Surrogate Key

Surrogate keys tend to be a compact data type, such as a four-byte integer. This allows the database to query the single key column faster than it could multiple columns. Furthermore, a non-redundant distribution of keys causes the resulting b-tree index to be completely balanced. Surrogate keys are also less expensive to join (fewer columns to compare) than compound keys.

Compatibility[edit]

While using several database application development systems, drivers, and object-relational mapping systems, such as Ruby on Rails or Hibernate, it is much easier to use an integer or GUID surrogate keys for every table instead of natural keys in order to support database-system-agnostic operations and object-to-row mapping.

Uniformity[edit]

When every table has a uniform surrogate key, some tasks can be easily automated by writing the code in a table-independent way.

Data Warehouse Surrogate Key Generation 2017

Validation[edit]

It is possible to design key-values that follow a well-known pattern or structure which can be automatically verified. For instance, the keys that are intended to be used in some column of some table might be designed to 'look differently from' those that are intended to be used in another column or table, thereby simplifying the detection of application errors in which the keys have been misplaced. However, this characteristic of the surrogate keys should never be used to drive any of the logic of the applications themselves, as this would violate the principles of Database normalization.

Disadvantages[edit]

Disassociation[edit]

The values of generated surrogate keys have no relationship to the real-world meaning of the data held in a row. When inspecting a row holding a foreign key reference to another table using a surrogate key, the meaning of the surrogate key's row cannot be discerned from the key itself. Every foreign key must be joined to see the related data item. If appropriate database constraints have not been set, or data imported from a legacy system where referential integrity was not employed, it is possible to have a foreign-key value that does not correspond to a primary-key value and is therefore invalid. (In this regard, C.J. Date regards the meaninglessness of surrogate keys as an advantage. ^[5])

To discover such errors, one must perform a query that uses a left outer join between the table with the foreign key and the table with the primary key, showing both key fields in addition to any fields required to distinguish the record; all invalid foreign-key values will have the primary-key column as NULL. The need to perform such a check is so common that Microsoft Access actually provides a 'Find Unmatched Query' wizard that generates the appropriate SQL after walking the user through a dialog. (It is, however, not too difficult to compose such queries manually.) 'Find Unmatched' queries are typically employed as part of a data cleansing process when inheriting legacy data.

Surrogate keys are unnatural for data that is exported and shared. A particular difficulty is that tables from two otherwise identical schemas (for example, a test schema and a development schema) can hold records that are equivalent in a business sense, but have different keys. This can be mitigated by NOT exporting surrogate keys, except as transient data (most obviously, in executing applications that have a 'live' connection to the database).

When surrogate keys supplant natural keys, then domain specific referential integrity will be compromised. For example, in a customer master table, the same customer may have multiple records under separate customer IDs, even though the natural key (a combination of customer name, date of birth, and E-mail address) would be unique. To prevent compromise, the natural key of the table must NOT be supplanted: it must be preserved as a unique constraint, which is implemented as a unique index on the combination of natural-key fields.

Query optimization[edit]

Relational databases assume a unique index is applied to a table's primary key. The unique index serves two purposes: (i) to enforce entity integrity, since primary key data must be unique across rows and (ii) to quickly search for rows when queried. Since surrogate keys replace a table's identifying attributes—the natural key—and since the identifying attributes are likely to be those queried, then the query optimizer is forced to perform a full table scan when fulfilling likely queries. The remedy to the full table scan is to apply indexes on the identifying attributes, or sets of them. Where such sets are themselves a candidate key, the index can be a unique index.

These additional indexes, however, will take up disk space and slow down inserts and deletes.

Normalization[edit]

Surrogate keys can result in duplicate values in any natural keys. To prevent duplication, one must preserve the role of the natural keys as unique constraints when defining the table using either SQL's CREATE TABLE statement or ALTER TABLE ..ADD CONSTRAINT statement, if the constraints are added as an afterthought.

Business process modeling[edit]

Because surrogate keys are unnatural, flaws can appear when modeling the business requirements. Business requirements, relying on the natural key, then need to be translated to the surrogate key. A strategy is to draw a clear distinction between the logical model (in which surrogate keys do not appear) and the physical implementation of that model, to ensure that the logical model is correct and reasonably well normalised, and to ensure that the physical model is a correct implementation of the logical model.

Inadvertent disclosure[edit]

Proprietary information can be leaked if sequential key generators are used. By subtracting a previously generated sequential key from a recently generated sequential key, one could learn the number of rows inserted during that time period. This could expose, for example, the number of transactions or new accounts per period. There are a few ways to overcome this problem:

• Increase the sequential number by a random amount.
• Generate a random key such as a UUID

Inadvertent assumptions[edit]

Sequentially generated surrogate keys can imply that events with a higher key value occurred after events with a lower value. This is not necessarily true, because such values do not guarantee time sequence as it is possible for inserts to fail and leave gaps which may be filled at a later time. If chronology is important then date and time must be separately recorded.

See also[edit]

References[edit]

Citations[edit]

1. ^'What is a Surrogate Key? - Definition from Techopedia'. Techopedia.com. Retrieved 2020-02-21.
2. ^P A V Hall, J Owlett, S J P Todd, 'Relations and Entities', Modelling in Data Base Management Systems (ed GM Nijssen),North Holland 1976.
3. ^http://docs.oracle.com/database/121/SQLRF/statements_7002.htm#SQLRF01402
4. ^https://msdn.microsoft.com/en-us/library/ff878091.aspx
5. ^ C.J. Date. The primacy of primary keys. From 'Relational Database Writings, 1991-1994. Addison-Wesley, Reading, MA.

Sources[edit]

Surrogate Key

• This article is based on material taken from the Free On-line Dictionary of Computing prior to 1 November 2008 and incorporated under the 'relicensing' terms of the GFDL, version 1.3 or later.

Data Warehouse Surrogate Key Generation Model

• Nijssen, G.M. (1976). Modelling in Data Base Management Systems. North-Holland Pub. Co. ISBN0-7204-0459-2.
• Engles, R.W.: (1972), A Tutorial on. CiteSeerX10.1.1.16.3195.Cite journal requires journal= (help)
• Date, C. J. (1998). 'Chapters 11 and 12'. Relational Database Writings 1994–1997. ISBN0201398141.
• Carter, Breck. 'Intelligent Versus Surrogate Keys'. Retrieved 2006-12-03.
• Richardson, Lee. 'Create Data Disaster: Avoid Unique Indexes – (Mistake 3 of 10)'. Archived from the original on 2008-01-30. Retrieved 2008-01-19.
• Berkus, Josh. 'Database Soup: Primary Keyvil, Part I'. Retrieved 2006-12-03.

What Is Surrogate Key