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Prezentace Database_Tvaroh Tomáš

Y04A2L - Anglický jazyk 2-2

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imary key) and they hardly occur, so if you manage to get your database into 3 NF, it can be considered normalized.

Another phase of database life cycle is creating a physical data model. Database, in this stage, has to be more specified in terms of database environment (selection between MySQL, Oracle, FireBird …) as well as in terms of further data restrictions. Such restrictions are knows as integrity constraints. Integrity constraints are rules ensuring consistency and validity of data. There are three fundamental constraints:

1 – entity constraint
Every row should be uniquely determined.

2 – domain constraint
In this part there is a need for specifying valid range of data to be inserted. In CDM you
said, that this column would be a number. In domain constraint you say that the number
should be between 1 and 100. It can be used if you want to store an age of somebody and
don’t want to have any 200 years old people. Another possibility is to restrict all kinds of
dates.

3 – referential constraint
This is, in my opinion, the most important of all integrity constraints. In the database there
are often records dependant on other records. If you have a discussion forum, you
probably have a table of forums and table of posts, where every post has a link to the
forum it belongs to. But what happens if you delete the forum and omit to delete all it‘s
posts? You would have redundant data that should have been deleted as well. Integrity
constraint define a link which says that if you delete a forum, all posts in that forum are
automatically deleted as well.

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Vloženo: 28.04.2009

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