Difference between revisions of "CALCULATED Column Constraint"
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==Purpose== | ==Purpose== | ||
Column constraint to calculate the value of a column | Column constraint to calculate the value of a column | ||
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==Syntax== | ==Syntax== | ||
CALCULATED <expr> | CALCULATED <expr> | ||
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==See Also== | ==See Also== | ||
[[ALTER TABLE]], [[SQL Constraints|CONSTRAINTS]], [[CREATE TABLE]] | [[ALTER TABLE]], [[SQL Constraints|CONSTRAINTS]], [[CREATE TABLE]] | ||
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==Description== | ==Description== | ||
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The CALCULATED column constraint is used to calculate the value of a column whenever it is accessed. The <expr> specified must evaluate to the same data type as the target column. Specifying the CALCULATED column constraint causes the column to be read only. If the <expr> is based on other columns from the same table, these columns must have the RECALCULATE column constraint set, so that any changes to their values cause calculated fields to be recalculated. | The CALCULATED column constraint is used to calculate the value of a column whenever it is accessed. The <expr> specified must evaluate to the same data type as the target column. Specifying the CALCULATED column constraint causes the column to be read only. If the <expr> is based on other columns from the same table, these columns must have the RECALCULATE column constraint set, so that any changes to their values cause calculated fields to be recalculated. | ||
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==Example== | ==Example== | ||
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[[Category:Documentation]] | [[Category:Documentation]] | ||
[[Category:SQL]] | [[Category:SQL]] | ||
Latest revision as of 07:36, 7 December 2012
Contents
Purpose
Column constraint to calculate the value of a column
Syntax
CALCULATED <expr>
See Also
ALTER TABLE, CONSTRAINTS, CREATE TABLE
Description
A constraint is used to define rules that help to provide data integrity. Column constraints are specific to the column name specified. You must have ALTER privilege on the table. The table will be locked for EXCLUSIVE use during the operation.
The CALCULATED column constraint is used to calculate the value of a column whenever it is accessed. The <expr> specified must evaluate to the same data type as the target column. Specifying the CALCULATED column constraint causes the column to be read only. If the <expr> is based on other columns from the same table, these columns must have the RECALCULATE column constraint set, so that any changes to their values cause calculated fields to be recalculated.
Example
ALTER TABLE customer; ALTER COLUMN available CALCULATED limit-balance; ALTER COLUMN limit RECALCULATE; ALTER COLUMN balance RECALCULATE
