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Prezentace Database_Tvaroh Tomáš

Y04A2L - Anglický jazyk 2-2

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sults in database inconsistencies.

Normal forms (NF):
Normal forms provide criteria for determining a table’s level of invulnerability to such anomalies. The higher the form is, the better.

1 NF – Every record in every single cell has to be atomic (it means that it has to have only
one value and not more). Every other table, which doesn’t meet this requirements, is considered to be in 0 NF.
NameAbilitiesTomas Tvarohenglish, piano, table tennisJan Novaksquash, spanish

*Example of the table in 0 NF.

Another common violation of 1 NF is keeping address or date in one cell. There is no standard way to neither of it and it may cause troubles. Moreover it consists of more elements (street, city, postal code ..)

2 NF - Each non-key attribute must depend on the whole primary key. It’s dealing only with
the tables with composite primary key (composed from more than one attribute). Tables in 1NF with a single primary key are automatically in 2NF. If your table contains such dependancies, you have to split it into two (or more) tables.

-> student_id student.idsubject_id
-> subject_id subject_idsubject_name
-> evaluation= evaluation+
-> evaluation_date evaluation_date
-> subject_name

3 NF – All attributes must depend on the primary key and not on each other. If they do, table
has to be divided into more tables and every new table should meet this requirement.

-> emploee_id emploee_idtask_id
-> emploee_name emploee_nametask_description
-> task_id= +emploee_id
-> task_description

There are 6 NF, but 4-6 are very complicated (they deal with relations between attributes forming the pr

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Vloženo: 28.04.2009

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