Ambiguous modifications to a view that has a subquery in the FROM clause?

Question

The SELECT statement cannot contain a subquery in the FROM clause.

If you modify a view that contains such a subquery, why is the modifcation of the base relation on which it depends ambiguous?

EDIT : I've come closer to finding a solution. If you have a table that contains, student ID's and names, and you want to create a view that contains all non-unique names, as in:

create view NonUnique as
select * from Student S1
where exists (select * from Student S2
              where S1.sID <> S2.sID
              and S2.name = S1.name)

then delete from NonUnique is specifying an ambiguous modifcation to the Student table, because you could clear out the view by deleting all students, or just a handful of students such that only uniquely named students remain.

Are there any other examples of ambiguous modifications we could make to a view that contains a subquery?

Answer 1

I don't see any ambiguity in the delete. DELETE FROM aView ; should delete (if allowed) all rows in the underlying table that are in the View.

CREATE TABLE Student
  ( sid INT NOT NULL
  , name VARCHAR(20) NOT NULL
  , PRIMARY KEY (sid)
  ) ;

INSERT INTO Student
  (sid, name)
VALUES
  (1, 'Alex'),
  (2, 'Bill'),
  (3, 'Cate'),
  (4, 'Dean'),
  (5, 'Eve'),
  (6, 'Alex'),
  (7, 'Bill'),
  (8, 'Cate') ;

CREATE VIEW NonUnique AS
SELECT * 
FROM Student S1
WHERE EXISTS 
      ( SELECT * 
        FROM Student S2
        WHERE S1.sID <> S2.sID
          AND  S2.name = S1.name
      ) ;

DELETE FROM NonUnique ;

It works fine in SQL products that have implemented DELETE correctly. See the SQL-Fiddles for SQL-Server and Oracle :

SELECT * FROM Student ;

Result:

sid | name
----------
  4 | Dean
  5 | Eve

---

MySQL provides error: The target table NonUnique of the DELETE is not updatable

Postgres gives: ERROR: cannot delete from view "nonunique":

---

If you try this in MySQL:

DELETE s 
FROM Student AS s
   JOIN 
     NonUnique AS n 
       ON n.sid = s.sid ;

or this:

DELETE
FROM Student 
WHERE EXISTS
      ( SELECT *
        FROM NonUnique 
        WHERE NonUnique.sid = Student.sid 
      ) ;

the error is: The definition of table 'NonUnique' prevents operation DELETE on table 'Student'.

---

In Postgres however, the second statement succeeds and deletes the correct rows.

MySQL succeeds only if you manage to hide the view inside a derived table:

DELETE s 
FROM Student AS s
   JOIN 
     ( SELECT *
       FROM NonUnique
     ) AS n 
     ON n.sid = s.sid ;

Answer 2

My instructor wrote this answer:

The SQL standard does not allow that view to be modified, because of the ambiguity.

@ypercube makes the point that there does exist a well-defined way to translate modifications to that view to modifications to the base table. However, there are other modifications to the base table that could similarly modify the view. Thus, I think "delete * from the-specificied-view" is actually ambiguous.