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
In what two ways will the composition of blood coming from the pulmonary artery vary from that going to the pulmonary vein? Blood in the pulmonary artery will have less oxygen

Concepts and Definitions in Relation to Nutrient Requirements 1) The probability concept describes the relationship between the levels of intake and the probability of risk of

Explain Healthy nutrition There is no doubt that adoption of Healthy nutrition lifestyle and the practice of good nutrition habits would help eliminate many health problems ca

what are the importance in studying animal tissues?

Although there is much talk in the news about stem cell research, the public and policymakers need to understand how basic body cells work to transmit information and replicate to

What is the valve that separates the stomach from the esophagus called? What is its function? The valve that separates the stomach from the esophagus is the cardia. It has the

Saturated fatty  acids General formula of saturated fatty acids  is CH3  (CHI), COOH. It has a polar end with a free XOOH group and a non-polar end with a hydrocarbon chain, as

In which areas of the globe is gymnosperm abundance noteworthy? These plants are the typical vegetation of cold regions as the taiga, or boreal forest, of the northern hemisph

Why is AIDS difficult to prevent by vaccination? It is complex to produce a vaccine against AIDS because the HIV is a highly mutant virus. In almost every replication the produ

You are working on a protocol in lab that requires 10ml of a 5% saline solution for a particular step. All you have in your lab is a 20% saline stock solution. How can you dilute t