MySQL 5.7 Reference Manual Including MySQL NDB Cluster 7.5 and NDB Cluster 7.6

13.1.18.8 CREATE TABLE and Generated Columns

CREATE TABLE supports the specification of generated columns. Values of a generated column are computed from an expression included in the column definition.

Generated columns are supported by the NDB storage engine beginning with MySQL NDB Cluster 7.5.3.

The following simple example shows a table that stores the lengths of the sides of right triangles in the sidea and sideb columns, and computes the length of the hypotenuse in sidec (the square root of the sums of the squares of the other sides):

CREATE TABLE triangle (
  sidea DOUBLE,
  sideb DOUBLE,
  sidec DOUBLE AS (SQRT(sidea * sidea + sideb * sideb))
);
INSERT INTO triangle (sidea, sideb) VALUES(1,1),(3,4),(6,8);

Selecting from the table yields this result:

mysql> SELECT * FROM triangle;
+-------+-------+--------------------+
| sidea | sideb | sidec              |
+-------+-------+--------------------+
|     1 |     1 | 1.4142135623730951 |
|     3 |     4 |                  5 |
|     6 |     8 |                 10 |
+-------+-------+--------------------+

Any application that uses the triangle table has access to the hypotenuse values without having to specify the expression that calculates them.

Generated column definitions have this syntax:

col_name data_type [GENERATED ALWAYS] AS (expr)
  [VIRTUAL | STORED] [NOT NULL | NULL]
  [UNIQUE [KEY]] [[PRIMARY] KEY]
  [COMMENT 'string']

AS (expr) indicates that the column is generated and defines the expression used to compute column values. AS may be preceded by GENERATED ALWAYS to make the generated nature of the column more explicit. Constructs that are permitted or prohibited in the expression are discussed later.

The VIRTUAL or STORED keyword indicates how column values are stored, which has implications for column use:

The default is VIRTUAL if neither keyword is specified.

It is permitted to mix VIRTUAL and STORED columns within a table.

Other attributes may be given to indicate whether the column is indexed or can be NULL, or provide a comment.

Generated column expressions must adhere to the following rules. An error occurs if an expression contains disallowed constructs.

If the expression evaluates to a data type that differs from the declared column type, implicit coercion to the declared type occurs according to the usual MySQL type-conversion rules. See Section 12.2, “Type Conversion in Expression Evaluation”.

Note

If any component of the expression depends on the SQL mode, different results may occur for different uses of the table unless the SQL mode is the same during all uses.

For CREATE TABLE ... LIKE, the destination table preserves generated column information from the original table.

For CREATE TABLE ... SELECT, the destination table does not preserve information about whether columns in the selected-from table are generated columns. The SELECT part of the statement cannot assign values to generated columns in the destination table.

Partitioning by generated columns is permitted. See Table Partitioning.

A foreign key constraint on a stored generated column cannot use ON UPDATE CASCADE, ON DELETE SET NULL, ON UPDATE SET NULL, ON DELETE SET DEFAULT, or ON UPDATE SET DEFAULT.

A foreign key constraint cannot reference a virtual generated column.

For InnoDB restrictions related to foreign keys and generated columns, see Section 14.6.1.5, “InnoDB and FOREIGN KEY Constraints”.

Triggers cannot use NEW.col_name or use OLD.col_name to refer to generated columns.

For INSERT, REPLACE, and UPDATE, if a generated column is inserted into, replaced, or updated explicitly, the only permitted value is DEFAULT.

A generated column in a view is considered updatable because it is possible to assign to it. However, if such a column is updated explicitly, the only permitted value is DEFAULT.

Generated columns have several use cases, such as these:

Example:

Suppose that a table t1 contains first_name and last_name columns and that applications frequently construct the full name using an expression like this:

SELECT CONCAT(first_name,' ',last_name) AS full_name FROM t1;

One way to avoid writing out the expression is to create a view v1 on t1, which simplifies applications by enabling them to select full_name directly without using an expression:

CREATE VIEW v1 AS
SELECT *, CONCAT(first_name,' ',last_name) AS full_name FROM t1;

SELECT full_name FROM v1;

A generated column also enables applications to select full_name directly without the need to define a view:

CREATE TABLE t1 (
  first_name VARCHAR(10),
  last_name VARCHAR(10),
  full_name VARCHAR(255) AS (CONCAT(first_name,' ',last_name))
);

SELECT full_name FROM t1;