HOME

TheInfoList



OR:

A surrogate key (or synthetic key, pseudokey, entity identifier, factless key, or technical key) in a
database In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and a ...
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.


Definition

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. ; 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 A data model is an abstract model that organizes elements of data and standardizes how they relate to one another and to the properties of real-world entities. For instance, a data model may specify that the data element representing a car be ...
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 A temporal database stores data relating to time instances. It offers temporal data types and stores information relating to past, present and future time. Temporal databases can be uni-temporal, bi-temporal or tri-temporal. More specifically the ...
. 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 have argued that a surrogate should have the following characteristics: * the value is 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

In a current database, the surrogate key can be the
primary key In the relational model of databases, a primary key is a designated attribute (column) that can reliably identify and distinguish between each individual record in a table. The database creator can choose an existing unique attribute or combinati ...
, generated by the
database management system In computing, a database is an organized collection of data or a type of data store based on the use of a database management system (DBMS), the software that interacts with end users, applications, and the database itself to capture and an ...
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 A Universally Unique Identifier (UUID) is a 128-bit nominal number, label used to uniquely identify objects in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems. When generated according to the ...
(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 Sybase, Inc. was an enterprise software and services company. The company produced software relating to relational databases, with facilities located in California and Massachusetts. Sybase was acquired by SAP in 2010; SAP ceased using the Syba ...
or SQL Server "identity column", a
PostgreSQL PostgreSQL ( ) also known as Postgres, is a free and open-source software, free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. PostgreSQL features transaction processing, transactions ...
or
Informix Informix is a product family within IBM's Information Management division that is centered on several relational database management system (RDBMS) and multi-model database offerings. The Informix products were originally developed by Inform ...
serial, an
Oracle An oracle is a person or thing considered to provide insight, wise counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. If done through occultic means, it is a form of divination. Descript ...
or SQL Server SEQUENCE or a column defined with AUTO_INCREMENT in
MySQL MySQL () is an Open-source software, open-source relational database management system (RDBMS). Its name is a combination of "My", the name of co-founder Michael Widenius's daughter My, and "SQL", the acronym for Structured Query Language. A rel ...
). Some databases provide
UUID A Universally Unique Identifier (UUID) is a 128-bit nominal number, label used to uniquely identify objects in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems. When generated according to the ...
/
GUID A Universally Unique Identifier (UUID) is a 128-bit label used to uniquely identify objects in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems. When generated according to the standard methods ...
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 A temporal database stores data relating to time instances. It offers temporal data types and stores information relating to past, present and future time. Temporal databases can be uni-temporal, bi-temporal or tri-temporal. More specifically the ...
, 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 Identifier A Universally Unique Identifier (UUID) is a 128-bit label used to uniquely identify objects in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems. When generated according to the standard methods ...
s (UUIDs) *
Globally Unique Identifier A Universally Unique Identifier (UUID) is a 128-bit label used to uniquely identify objects in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems. When generated according to the standard methods ...
s (GUIDs) * Object Identifiers (OIDs) *
Sybase Sybase, Inc. was an enterprise software and services company. The company produced software relating to relational databases, with facilities located in California and Massachusetts. Sybase was acquired by SAP in 2010; SAP ceased using the Syba ...
or SQL Server identity column IDENTITY OR IDENTITY(n,n) *
Oracle An oracle is a person or thing considered to provide insight, wise counsel or prophetic predictions, most notably including precognition of the future, inspired by deities. If done through occultic means, it is a form of divination. Descript ...
SEQUENCE, or GENERATED AS IDENTITY (starting from version 12.1) * SQL Server SEQUENCE (starting from SQL Server 2012) *
PostgreSQL PostgreSQL ( ) also known as Postgres, is a free and open-source software, free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. PostgreSQL features transaction processing, transactions ...
or IBM Informix serial * MySQL AUTO_INCREMENT * SQLite INTEGER PRIMARY KEY (if AUTOINCREMENT is used it will prevent the reuse of numbers that have already been used but are available) * AutoNumber data type in
Microsoft Access Microsoft Access is a database management system (DBMS) from Microsoft that combines the relational database, relational Access Database Engine (ACE) with a graphical user interface and software-development tools. It is a member of the Microsof ...
* AS IDENTITY GENERATED BY DEFAULT in IBM Db2 and
PostgreSQL PostgreSQL ( ) also known as Postgres, is a free and open-source software, free and open-source relational database management system (RDBMS) emphasizing extensibility and SQL compliance. PostgreSQL features transaction processing, transactions ...
. * Identity column (implemented in DDL) in
Teradata Teradata Corporation is an American software company that provides cloud database and Analytics, analytics-related software, products, and services. The company was formed in 1979 in Brentwood, California, as a collaboration between researchers a ...
* Table Sequence when the sequence is calculated by a procedure and a sequence table with fields: id, sequenceName, sequenceValue and incrementValue


Advantages


Stability

Surrogate keys typically 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 does not change). * The primary or natural key data can always be modified, even with databases that do not support cascading updates across related
foreign key A foreign key is a set of attributes in a table that refers to the primary key of another table, linking these two tables. In the context of relational databases, a foreign key is subject to an inclusion dependency constraint that the tuples ...
s.


Requirement changes

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

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 (which are often text - which is slower still). Furthermore, a non-redundant distribution of keys causes the resulting
b-tree In computer science, a B-tree is a self-balancing tree data structure that maintains sorted data and allows searches, sequential access, insertions, and deletions in logarithmic time. The B-tree generalizes the binary search tree, allowing fo ...
index to be completely balanced. Surrogate keys are also less expensive to join (fewer columns to compare) than compound key.


Compatibility

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

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


Validation

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 Database normalization is the process of structuring a relational database in accordance with a series of so-called '' normal forms'' in order to reduce data redundancy and improve data integrity. It was first proposed by British computer scien ...
.


Simplicity of Relationships

Surrogate keys simplify the creation of foreign key relationships because they only require a single column (as opposed to composite keys - which require multiple columns). When creating a query on the database, forgetting to include all the columns in a composite foreign key when joining tables can lead to unexpected results in the form of an undesired
cartesian product In mathematics, specifically set theory, the Cartesian product of two sets and , denoted , is the set of all ordered pairs where is an element of and is an element of . In terms of set-builder notation, that is A\times B = \. A table c ...
.


Disadvantages


Disassociation

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.C.J. Date. The primacy of primary keys. From "Relational Database Writings, 1991-1994. Addison-Wesley, Reading, MA.) 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 Data cleansing or data cleaning is the process of identifying and correcting (or removing) corrupt, inaccurate, or irrelevant records from a dataset, table, or database. It involves detecting incomplete, incorrect, or inaccurate parts of the dat ...
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

Relational databases assume a unique
index Index (: indexes or indices) may refer to: Arts, entertainment, and media Fictional entities * Index (''A Certain Magical Index''), a character in the light novel series ''A Certain Magical Index'' * The Index, an item on the Halo Array in the ...
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

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

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

Proprietary information can be leaked if surrogate keys are generated sequentially. 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. For example see German tank problem. 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 A Universally Unique Identifier (UUID) is a 128-bit nominal number, label used to uniquely identify objects in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems. When generated according to the ...
.


Inadvertent assumptions

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

* Natural key * Object identifier * Persistent object identifier


References


Citations


Sources

* * Engles, R.W.: (1972), ''A Tutorial on Data-Base Organization'', Annual Review in Automatic Programming, Vol.7, Part 1, Pergamon Press, Oxford, pp. 1–64. * Langefors, B (1968). ''Elementary Files and Elementary File Records'', Proceedings of File 68, an IFIP/IAG International Seminar on File Organisation, Amsterdam, November, pp. 89–96. * * * * * {{DEFAULTSORT:Surrogate Key Data modeling Database management systems