The SYS_KEY_COLUMN_USAGE table contains information to match columns with constraints. Each primary key has one column, _id. It has the following columns:

Column Name	Data Type	Description
CONSTRAINT_CATALOG	VARCHAR	The name of the database of the connection.
CONSTRAINT_SCHEMA	VARCHAR	Not used in dotConnect for MongoDB. Always NULL.
CONSTRAINT_NAME	VARCHAR	The name of the constraint.
TABLE_CATALOG	VARCHAR	The name of the database of the connection.
TABLE_SCHEMA	VARCHAR	Not used in dotConnect for MongoDB. Always NULL.
TABLE_NAME	VARCHAR	The name of the collection which the column belongs to.
COLUMN_NAME	VARCHAR	The name of the column. Always _id.
ORDINAL_POSITION	INT	The position of the column in the key. Always 1.