Sql''s fifth concept - null, Biology

Assignment Help:

SQL's Fifth Concept - NULL

Unfortunately, SQL embraces a fifth concept, called NULL, an apparently simple little thing but one that has pervasive effects on our usual understanding of the other four. NULL denotes a kind of nothingness. It is somewhat akin to a value in that it can be the result of evaluating an expression, but it has no type, cannot appear everywhere a value can appear, and lacks certain other properties that values have, as we shall see.

Although SQL is not the only computer language to include a special construct representing non- existence, its own variety, NULL is accompanied by a departure from classical logic that is not found in any other well-known languages and gives rise to some deviations from relational database theory that students and users have to be well aware of. The departure in question is the introduction of a third truth value, denoted by the key word UNKNOWN. For example, a comparison of the form x = y in SQL evaluates to UNKNOWN if either x or y evaluates to NULL, even when they both do! (The comparison operator IS NOT DISTINCT FROM is available in place of =, such that x IS NOT DISTINCT FROM y evaluates to FALSE if just one of x and y is NULL, TRUE if they both are, and is otherwise equivalent to x = y.)

 


Related Discussions:- Sql''s fifth concept - null

Why is the development of bipedalism considered, Why is the development of ...

Why is the development of bipedalism considered to be a major advancement in human evolution?

Explain the organisation of the intensive care unit, Explain the organisati...

Explain the organisation of the intensive care unit? The following kinds of professional groups are required in the day to day functioning of a modern intensive care unit: Th

Surgery for coronary artery disease, SURGERY FOR CORONARY ARTERY DISEASE : ...

SURGERY FOR CORONARY ARTERY DISEASE : Stenotic coronary artery disease (CAD) is caused by the thickening and narrowing of the coronary arteries (Atherosclerosis). Initially it cau

Define fat needs in nutrient requirement, Define Fat needs in Nutrient Requ...

Define Fat needs in Nutrient Requirement and Dietary Management? As we have learnt earlier on enteral and parenteral feedings; administration of lipids should be carried out ca

Estrus - estrous cycle, Estrus - Estrous cycle This is the period of h...

Estrus - Estrous cycle This is the period of heat, and copulation is permitted only at this time. This condition lasts from 9 to 15 hours and is characterised by a high rate o

Explain the selective media - culture media, Explain the Selective Media - ...

Explain the Selective Media - Culture Media? It is used to select specific groups of bacteria by favouring the growth of desired bacteria and inhibiting the growth of undesired

Explain the importance of human genome project, Question 1: Explain the...

Question 1: Explain the importance of Human Genome Project in Research and list the different methods available for Gene sequencing Mention the importance of Human Genome

Respiration, difference between anaerobic respiration and fermentation

difference between anaerobic respiration and fermentation

Phylum protozoa, diversification and economic importance

diversification and economic importance

Parasympathetic discharge to the heart leads to, An enhance in parasympathe...

An enhance in parasympathetic discharge to the heart leads to A. An enhance in the conductance of F-channels in SA node cells. B. An enhance in the conductance of potassium

Write Your Message!

Captcha
Free Assignment Quote

Assured A++ Grade

Get guaranteed satisfaction & time on delivery in every assignment order you paid with us! We ensure premium quality solution document along with free turntin report!

All rights reserved! Copyrights ©2019-2020 ExpertsMind IT Educational Pvt Ltd