December 2014
Intermediate to advanced
852 pages
17h 57m
English
Content preview from Joe Celko's SQL for Smarties, 5th Edition
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Chapter 14
NULLs—Missing Data in SQL
Abstract
NULLs are how we handle missing and unknown data. They can be in the original data or generated by functions in SQL. NULLs propagate in math and logic.
Keywords
NULL
Dr. E. F. Codd
Fact “real NULLs”
“Created NULLs”
Empty table
Logic
3VL
Łukasiewicz
COALESCE()
NULLIF()
INDICATOR in host language
FOREIGN KEY
A discussion of how to handle missing data enters a sensitive area in relational database circles. Dr. E. F. Codd, creator of the relational model, favored two types of missing-value tokens in his book on the second version of the relational model, one for “unknown” (the eye color of a man wearing sunglasses) and one for “not applicable” (the eye color of an automobile). Chris Date, leading ...
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