Knobit of Data modelling · K-629 · Skill

Recognises a sentinel value standing in for null

What it means

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.

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

Builds on

Recognises an attribute with several meaningsK-506 · Skill
Explains null semantics and three-valued logicK-655 · Knowledge

Needed by

Recognises a default inventing a business factK-820 · Skill