My best guess is that it's sort of like the heirarchical table format (FK referencing PK from the same table).
CREATE TABLE test_dim
(
test_key NUMBER(10,0) PRIMARY KEY,
source VARCHAR2(10) NOT NULL,
source_subcategory VARCHAR2(10)
);
INSERT INTO test_dim( test_key, source, source_subcategory )
VALUES ( 1, 'MEDICAID', NULL );
INSERT INTO test_dim( test_key, source, source_subcategory )
VALUES ( 2, 'MEDICARE', NULL );
INSERT INTO test_dim( test_key, source, source_subcategory )
VALUES ( 3, 'SSA', 'MEDICARE' );
INSERT INTO test_dim( test_key, source, source_subcategory )
VALUES ( 4, 'SPAP', 'MEDICARE' );
INSERT INTO test_dim( test_key, source, source_subcategory )
VALUES ( 5, 'MAPD', 'MEDICARE' );
CREATE DIMENSION test_dim
LEVEL source IS ( test_dim.source )
LEVEL source_subcategory IS ( test_dim.source_subcategory );
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