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31/post-1/ - ذخیره شده نمایش نتایج بیشتر از farsi1tv.17.مشخصات فرکانس جدید کانال فارسی ۱ – Farsi 1 27 آگوست 2010 ..., those created to meet a writer's need at a particular moment.Most compounds whose meanings are self–evident from the meanings of their component words also will not be listed in the dictionary, even if they are permanent and widely used.Boyce, Codd, Date et al refer to these in the Relational Model as Candidate Keys.When we then build tables for these Entities their Candidate Keys become Natural Keys in those tables.

When set as the clustering key, so as to be used for key lookups into the clustered index from non-clustered indexes and foreign key lookups from related tables, all these disadvantages disappear.DROP TABLE IF EXISTS `test`.`animals`; CREATE TABLE `test`.`animals` ( `grp` char(30) NOT NULL, `id` mediumint(9) NOT NULL AUTO_INCREMENT, `name` char(30) NOT NULL, PRIMARY KEY (`grp`,`id`) ) ENGINE=My ISAM; INSERT INTO animals (grp,name) VALUES ('mammal','dog'),('mammal','cat'), ('bird','penguin'),('fish','lax'),('mammal','whale'), ('bird','ostrich'); SELECT * FROM animals ORDER BY grp,id; Which returns: -------- ---- --------- | grp | id | name | -------- ---- --------- | fish | 1 | lax | | mammal | 1 | dog | | mammal | 2 | cat | | mammal | 3 | whale | | bird | 1 | penguin | | bird | 2 | ostrich | -------- ---- --------- Primary Key is very unfortunate notation, because of the connotation of "Primary" and the subconscious association in consequence with the Logical Model. Instead I refer to the Surrogate Key of the Physical Model and the Natural Key(s) of the Logical Model.It is important that the Logical Model for every Entity have at least one set of "business attributes" which comprise a Key for the entity.However the Physical Model for our tables will in many instances be inefficient without a Surrogate Key.Recall that non-covered columns for a non-clustered index can only be found (in general) through a Key Lookup into the clustered index (ignore tables implemented as heaps for a moment).

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