PL-300 · 2.1.3 · Model the data
Cardinality and cross filter
Define a relationship's cardinality and cross-filter direction
defines uniqueness; defines how filters propagate between related tables.
Cardinality is about uniqueness
describes whether the columns contain unique or repeated values. In a , the dimension key is normally unique and the repeats that key.
Recognize the patterns
| Pattern | Typical meaning |
|---|---|
| One-to-many | Dimension filters a |
| One-to-one | Both sides contain unique values |
| Many-to-many | Both sides can repeat values |
Many-to-many is valid, but it needs more care because filter behavior can be less obvious.
Cross-filter direction
Single direction from dimension to fact is the safest default. Use Both only when the model genuinely needs filters to travel both ways, because extra filter paths can create ambiguity.
Exam clue: determine uniqueness first, then decide how filters must propagate.
Key points
- One-to-many: standard star-schema
- : depends on unique and duplicate values
- : controls filter propagation direction
- Both direction: can create ambiguity and slower models
Click an underlined term for a one-line explanation, or see them all in the glossary .
Exam trap
Changing changes valid filter directions; it is not merely a label change.
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