Using Caché Objects - InterSystems Documentation

Objects and SQL
• All the columns in the projection of the superclass (that is, those based on all the properties
in its superclass' extent)
• Additional columns based on properties only in the subclass
• A subset of the rows in the superclass' table that consists only of the instances of the
subclass
For example, the projection of a persistent class called Sample.Employee that is derived from
the Sample.Person class is a table containing all the fields defined by both the
Sample.Employee and Sample.Person classes. This is illustrated in the following SQL queries.
First list all instances of Sample.Person and its properties:
SELECT * FROM Sample.Person
Now list all instances of Sample.Employee and its properties:
SELECT * FROM Sample.Employee
Typically, the table of a subclass has more columns and fewer rows than its parent. There
are more columns in the subclass since it usually adds additional properties when it extends
the parent class; there are often fewer rows since there are often fewer instances of the subclass
than the parent.
13.1.1 How a Class is Projected to SQL
To create a class that is automatically accessible from SQL, define it based on the %Persistent.
When you compile the class, the Caché Class Compiler automatically generates the runtime
information needed for relational access. The class' instances are then available as the rows
of a relational table.
13.1.2 Naming Rules for Projected Classes
Caché itself places no restrictions on class names. However, SQL tables cannot have names
that are SQL Reserved Words; hence, if you create a persistent class with a name that is a
reserved word, the Caché Class Compiler will generate an error message. In this case, you
must either rename the class or specify a table name for the projection that differs from the
class name; to do this, use the SQLTABLENAME keyword, which has the following syntax:
SQLTABLENAME = TableNameForSQL;
114 Using Caché Objects