Typical mistakes at the solution of SELECT statement exercises

Database "Computer firm"

Find all the makers who have all their models of PC type in the PC table

Here is the typical fallacious query

Console

Execute

SELECT DISTINCT maker
FROM Product
WHERE model IN (SELECT model FROM PC);

SELECT DISTINCT maker FROM Product WHERE model IN (SELECT model FROM PC);

    Scheme

The key point of the claim is the word «ALL». Let’s analyze the models of Producer E. For PC models, issued by Producer E, we can generate the following query:

Console

Execute

SELECT model 
FROM Product 
WHERE maker='E' AND type='PC';

SELECT model FROM Product WHERE maker='E' AND type='PC';

    Scheme

Result:

And now let’s see, which of the models are provided in PC table:

Console

Execute

SELECT DISTINCT model 
FROM PC 
WHERE model IN(1260, 2111, 2112);

SELECT DISTINCT model FROM PC WHERE model IN(1260, 2111, 2112);

    Scheme

It appears that out of the three models only one – 1260 – is present in PC table. In accordance with the terms of the problem there should be ALL the three models of producer E.

Namely, solution of this problem boils down to operation of relational division, but for each producer there should be his own factor (set of the models). In the simplified way the operation of relational division can be recorded in the following way:

A(a, b) DIVIDEBY B(b)

Operation of relational division is rather primitive. It means that this operation can be represented using other (primitive) relational operations. Redundancy of relational algebra, suggested by Codd, is predetermined by its focus on practical application.  SQL(Structured Query Language) is a database computer language designed for the retrieval and management of data in relational database management systems (RDBMS), database schema creation and modification, and database object access control management.SQL language is also redundant, which is proved by every exercise, which can be solved in different ways. However it does not provide any analogue for relational division. :-)

To conclude with I would like to present relational division by means of other operations.

A DIVIDEBY B := 
A[a] EXCEPT ((A[a] TIMES B) EXCEPT A) [a]

In this respect A[a] means projection of relation A on attribute a; TIMES is Cartesian product. «Verbal» translation of the right part of the equation into SQL language can be represented in the following way:

Console

Execute

SELECT a FROM A
EXCEPT
SELECT a FROM (
SELECT A.a, B.b FROM A, B
EXCEPT
SELECT a,b FROM A) X;

SELECT a FROM A EXCEPT SELECT a FROM ( SELECT A.a, B.b FROM A, B EXCEPT SELECT a,b FROM A) X;

    Scheme

No scheme

One should not use this translation as guide to operations; there are some easier ways to solve this problem. However I am not very persistent in this respect. :-)

To solve the problem on SQL-EX.RU