On Mon, Feb 9, 2009 at 3:41 PM, Steven Buehler wrote: > Ok, I just saw a post about using view's in mysql. I tried to look it up > and found how to use it, but my question is: what is a view and why would > you use it?
The problem with any definition of an object in a database is that there are multiple definitions. Usually on the one hand you have the definition from abstract relational theory, and on the other hand you have the definition from actual working databases. So I am not going to bother with a definition, I will try to explain how a view works internally inside database code. The easiest way to understand a view is to consider a view as a macro that gets expanded during the execution of every query that references that view in its FROM. Lets take for example the view that your DBA has defined for you using: CREATE VIEW x AS SELECT * FROM y INNER JOIN z ON y.id = z.id; Then you query that view with the query: SELECT a FROM x; What the database will do for you behind the scenes is expand your usage of the view. In effect, the database will replace "x" with its definition. So your query SELECT a FROM x; gets expanded to: SELECT a FROM (SELECT * FROM y INNER JOIN z ON y.id = z.id); Notice that I have done nothing but replace x with its definition between parenthesis. And this results in a valid query that can be executed. And that is exactly what the database will do. It will do this substitution and then it will run the result of that substitution as if it were the query that you submitted. Obviously a bit more will go on behind the scenes to handle things like permissions and optimizations (especially if you get to databases that have more functionality then MySQL), but this is really all there is to it. A view is a simple macro that assigns an alias to a select statement, and when you reference that alias the select statement will get substituted back in. Jochem -- Jochem van Dieten http://jochem.vandieten.net/ -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/mysql?unsub=arch...@jab.org
