Sql''s fifth concept - null, Biology

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.)

 

Posted Date: 1/18/2013 1:33:10 AM | Location : United States







Related Discussions:- Sql''s fifth concept - null, Assignment Help, Ask Question on Sql''s fifth concept - null, Get Answer, Expert's Help, Sql''s fifth concept - null Discussions

Write discussion on Sql''s fifth concept - null
Your posts are moderated
Related Questions
Explain the Integumentary System in human biology? The skin, its glands, and outgrowths form the Integumentary system. This system provides protection, sensory perception, tem

Explain Adverse Effects of foscarnet  Renal dysfunction often develops during treatment with foscarnet and is usually reversible, but renal failure requiring dialy sis may occu

Short-Day Plants (SDP) - Plant Responses to Light-Dark Cycles These plants only flower or flower more profusely and rapidly when given less than a certain (critical) number of

Calculate Temperature for Survival and Growth of Microorganisms? Survival and growth of microorganisms is also affected by the temperature. According to their temperature requi

Describe the general examination of clinical examination? It is always to better to ask the patient to help himself/herself in setting on to the examination table and rename so

I have 9 questions in my undergraduate cardiovascular physiology class I need help with. It is mostly dealing with slow response action. I need the work in about 2nite hours from n

Vascular Cambium The vascular cambium usually, if not always, has a dual origin within the primary tissues; from provascular strands, and from the "ground" meristem tissues be

Normal 0 false false false EN-IN X-NONE X-NONE MicrosoftInternetExplorer4

Normal 0 false false false EN-US X-NONE X-NONE MicrosoftInternetExplorer4

The law of contract is that the branch of law which determines the circumstances in which promises made by the parities to a contract shall be legally binding on them. Its rules de