SQL- duplication of values from a query

Question

Assuming that company name and product name are primary keys at their relation, I don't understand how come that name of company can't be repeated in this query:

     SELECT  Company.name
     FROM    Company, Product
     WHERE   Company.name=Product.maker
              AND  Product.name  IN
                         (SELECT Purchase.product
                           FROM   Purchase
                           WHERE Purchase.buyer = ‘Joe Blow‘);

If S company made two products X,Y which Joe Blow bought them both, so at the beginning we would make a cartesian product for the condition "Company.name=Product.maker": SX, SY and for both, Product.name is in the set which compute, and therefor S should be output twice, no?

Answer 1

Without seeing your table structure or an error message, I can't really tell what's wrong. Your syntax appears to be correct.

However, your query could be improved by using better joins:

SELECT
    c.name
FROM
    Company c
    INNER JOIN Product prod
        ON c.name = prod.maker
    INNER JOIN Purchase pch
        ON prod.name = pch.product
WHERE
    pch.buyer = 'Joe Blow'

I would recommend using explicit joins ( INNER JOIN with an ON join condition) instead of implicit joins (comma-separated tables and join conditions in the WHERE clause) because it makes it clearer exactly how the tables are joined together.

Answer 2

I'd say, in majority of RDBMS your query will return two 'S'.

you can try it yourself here SQL FIDDLE

you can put distinct to have no duplicates

 SELECT  distinct Company.name
 FROM    Company, Product
 WHERE   Company.name=Product.maker
          AND  Product.name  IN
                     (SELECT Purchase.product
                       FROM   Purchase
                       WHERE Purchase.buyer = 'Joe Blow')

Answer 3

返回两个S是可以的，因为您从返回公司和产品的查询中预测公司名称，此查询的每一行都是唯一的，但是当您投影某些列时它们可能不是唯一的，或者可以使用DISTINCT