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Virtual column

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In relational databases a virtual column is a table column whose value(s) is automatically computed using other columns values, or another deterministic expression. Virtual columns are defined of SQL:2003 as Generated Column,[1] and are only implemented by some DBMSs, like MariaDB, SQL Server, Oracle, PostgreSQL, SQLite and Firebird (database server) (COMPUTED BY syntax).

Implementation

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There are two types of virtual columns:

  • Virtual columns
  • Persistent columns

Virtual columns values are computed on the fly when needed, for example when they are returned by a SELECT statement. Persistent column values are computed when a row is inserted in a table, and they are written like all other values. They can change if other values change. Both virtual and persistent columns have advantages and disadvantages: virtual columns don't consume space on the disk, but they must be computed every time a query refers to them; persistent columns don't require any CPU time, but they consume disk space. However, sometimes a choice is not available, because some DBMS's support only one column type (or neither of them).

IBM Db2

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IBM Db2 supports Virtual column of Version 8 as Generated column.[2]

MariaDB

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MariaDB is a MySQL fork. Virtual columns were added in the 5.2 tree.[3]

Expressions that can be used to compute the virtual columns have the following limitations:

  • They must be deterministic
  • They cannot return constant values
  • They cannot use user-defined functions or stored procedures
  • They cannot include other virtual columns
  • They cannot make use of subqueries

Persistent columns can be indexed and can be part of a foreign key, with a few small limitations concerning constraint enforcement.

Virtual columns can only be used on tables which use a storage engine which supports them. Storage engines supporting virtual columns are:

MRG_MyISAM tables can be based on MyISAM tables which include persistent columns; but the corresponding MRG_MyISAM column should be defined as a regular column.

Syntax

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A CREATE TABLE or ALTER TABLE statement can be used to add a virtual column. The syntax used to define a virtual column is the following:

<type>  [GENERATED ALWAYS]  AS   ( <expression> )  [VIRTUAL | PERSISTENT]  [UNIQUE] [UNIQUE KEY] [COMMENT <text>]
  • type is the column's data type
  • expression is the SQL expression which returns the column's value for each row
  • text is an optional column comment

MySQL

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Support for virtual columns, known in MySQL as generated columns, started becoming available in MySQL 5.7. Various limitations on their use have been relaxed in subsequent versions. [4]

Oracle

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Since version 11g, Oracle supports virtual columns.[5]

SQL Server

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Microsoft SQL Server supports virtual columns, but they are called Computed Columns.[6]

SQL Server supports both persisted and non-persisted computed columns.

Firebird

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Firebird has always supported virtual columns as its precursor InterBase supports it, called Computed Columns.[7]

Firebird supports virtual columns, not persistent ones and allows for sub-selects, calling built in functions, external functions and stored routines in the virtual column expression.

Syntax

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Creating a virtual column can be done during table creation or when adding columns to an existing table. The syntax used to define a virtual column is the following:

column_name [type] COMPUTED BY (expression)

or the industry standard

column_name [type] GENERATED ALWAYS AS (expression)

PostgreSQL

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Since version 12, PostgreSQL supports virtual columns, known as generated columns.[8]

SQLite

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Since version 3.31.0 (2020-01-22), SQLite supports virtual columns, known as generated columns.[9]

See also

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References

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  1. ^ SQL:2003 Has Been Published
  2. ^ SQL Reference Volume 2 Version 8
  3. ^ Virtual Columns
  4. ^ "MySQL :: MySQL 5.7 Reference Manual :: 13.1.18.7 CREATE TABLE and Generated Columns".
  5. ^ "Oracle 11g Schema Management". Archived from the original on 2017-08-06.
  6. ^ "Computed Columns". 4 October 2012.
  7. ^ "TABLE".
  8. ^ "5.3. Generated Columns". 12 August 2021.
  9. ^ "Generated Columns".
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