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Updateable Views

 
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Updateable Views
birdofprey
post 24 Jul, 2008 - 09:42 AM
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Hello Coders,

I've been reading this database book for my upcoming class this Fall and I've gotten up to Views.

I know that Views can be made updatable, always, by using triggers.

In the book they state that for Views to be updatable without using triggers are when:

1) The WHERE clause must not involve Relation R (Table) in the subquery.

- Does this mean that the view can't contain something like this
Select * From R WHERE (Select a FROM R Where id=1)?


2)The FROM clause only consist of one occurrence of R and no other relation.

- don't understand what this mean. Anyone could explain?
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AdaHacker
post 28 Jul, 2008 - 03:34 PM
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QUOTE(birdofprey @ 24 Jul, 2008 - 11:42 AM) *

1) The WHERE clause must not involve Relation R (Table) in the subquery.

- Does this mean that the view can't contain something like this
Select * From R WHERE (Select a FROM R Where id=1)?

That sounds about right to me.

QUOTE(birdofprey @ 24 Jul, 2008 - 11:42 AM) *

2)The FROM clause only consist of one occurrence of R and no other relation.

- don't understand what this mean. Anyone could explain?

Basically, it means "no joins in the FROM clause." The general principle is that for a view to be updatable, you must be able to directly map each row of the view to a row of the base table. If you your view is built from more than one table, it can introduce ambiguity into which rows of which table should be updated.
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birdofprey
post 29 Jul, 2008 - 04:19 PM
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thanks, how do i rep on this forum because you deserve one.
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