Virtual column

In relational databases a virtual column is a table column whose value 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[]

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[]

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

MariaDB[]

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:

• InnoDB
• MyISAM
• Aria

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[]

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[]

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[]

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

SQL Server[]

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

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

Firebird[]

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[]

Creating a virtual column can be done during table creation and 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[]

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

SQLite[]

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

Notes[]

External links[]

• Virtual Columns in MariaDB's documentation.
• MariaDB 5.2: What would you use virtual columns for? on OpenLife.cc
• Virtual Columns in Oracle Database 11g Release 1
• Computed Columns in SQL Server 2008