What is meant by 'query modification' as an approach to implementing views?

Just doing some revision, and one of the questions is:

"Explain what is meant by 'query modification' as an approach to implementing views."

Now, I'm not quite sure how to answer that... I know wh开发者_StackOverflow中文版at views are, how to create them and why they are used etc, but what exactly does that question want to know?

---

This is a theoretical concept from David Meier's works on relational theory.

When you are using a view in your queries, like this:

CREATE VIEW v_filtered
AS
SELECT  *
FROM    mytable
WHERE   mycolumn = 1

SELECT  *
FROM    v_filtered
JOIN    othertable
ON      otherid = myid

, to execute your query, a database engine should be able to rewrite the query over the virtual relations (like your view) to one using the base relations, since that what is actually stored:

SELECT  *
FROM    mytable
JOIN    othertable
ON      otherid = myid
WHERE   mycolumn = 1

This process is called query modification.