Values of computed columns are automatically maintained by the database server as rows are inserted and updated. Most applications should never need to update or insert computed column values directly; however, since computed columns are base columns like any other, they can be directly referenced in predicates and in expressions when it makes sense to do so.
Computed columns are not recalculated when queried. Instead, computed columns are recalculated under the following circumstances:
Any column is deleted, added, or renamed.
The table is renamed.
Any column's data type or COMPUTE clause is modified.
A row is inserted.
A row is updated.
If you use a time-dependent expression, or one that depends on the state of the database in some other way, then the computed column can not give a proper result.