Using Caché Objects - InterSystems Documentation

If the list for a particular instance contains no elements, it is projected as an empty string (and
not an SQL NULL value).
13.2.2.8 Stream Properties
Both character stream properties and binary stream properties are projected as BLOBs (binary
large objects).
Refer to the chapter on Streams for more information.
13.2.2.9 Transient and Calculated Properties
Since transient and calculated properties are not stored on disk, by default, they have no SQL
projection.
For the projection to include these columns, use the SQLCOMPUTED keyword, which allows
you to specify the code that calculates the value of an SQL computed field for the property;
you can then add code to the property definition that calculates the value of the projected
field. A property projected as a computed field can provide parallel object and SQL functionality
through parallel computations.
There are two ways to include code for a property's SQL computed field:
• Write code that performs the calculations required to determine the computed field's
value and associate it with the SQLCOMPUTECODE keyword.
• Write code that performs the calculations required to determine the computed field's
value and place it in a class method. Call this class method both from the code associated
with the SQLCOMPUTECODE keyword (for the SQL projection) and from the property's
Get method (for the object itself).
With either approach, this code can include values based on other properties and systembased
values. It can invoke class methods and user-defined functions.
Two Types of Computed SQL Fields
The Object-SQL Projection
The kind of computed SQL field described above is only one of the two types of computed
SQL fields:
• Always Computed (described above) — The field value is not stored in the database, but
calculated when needed upon retrieval.
• Triggered Computed — The field value is stored in the database and is calculated upon
INSERT/UPDATE if needed.
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