PrimaryKey (Index Keyword)
To specify that the primary key for this table is formed by the properties on which this index is based, use the following syntax:
Index name On property_expression_list [ PrimaryKey ];
Otherwise, omit this keyword or place the word Not immediately before the keyword.
This keyword specifies that this index should be reported, via SQL, as being the primary key for this class (table).
A PrimaryKey index also behaves like a unique index. That is, for the property (or the combination of properties) that you use in this index, InterSystems IRIS enforces uniqueness. It is permitted, but redundant, to specify the Unique keyword as true in this index definition.
Index EmpIDX On EmployeeID [ PrimaryKey] ;
If you omit this keyword, the primary key for this table is not formed by the properties on which this index is based.