Recognises a null with a business meaning
A real-looking value standing for "unknown" or "not applicable", or a null that means something specific and different in different tables. Sentinels pass every constraint and end up in totals and averages. A specific form of logical row 6 at the physical level.
Can AI do this?
Not tested yet
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What it looks like in practice
Expected result: The null, its meaning, and the explicit value or column stating it
How it is assessed: Flags the deficient case and passes the acceptable one. Deficient: account_opening_date = '1900-01-01' when unknown; branch_id = -1; open-ended validity as NULL in one table and 9999-12-31 in another Acceptable: NULL for unknown. Where a value is required, one project-wide convention recorded in config (open-ended valid_to), and a dimension's unknown member as a labelled row that every query sees, not a magic number in a measure
Also known as: Sentinel value in place of null, or null with a business meaning (physical model)
Related knobits
Builds on
- K-506 Recognises an attribute with several meanings
- K-655 Explains null semantics and three-valued logic
All 40 knobits of Design the physical data model.
Source: Review checklist 06-physical, row 19; Karwin, Fear of the Unknown; Codd 1979 §2.3