Execute a query for each value returned by subquery

Question

Hy, I have a query and a subquery(both of them are selects) and I want the outer query to be executed for each value returned by the subquery but the outer query has to return only one result.
For example, the following query is right for what I want?:
the following query finds out which authors live in the same city by looking at the postal code:
select au_fname, au_lname, city
from authors
where city = all
(select city
from authors
where postalcode like "946%")
I undertand the sentence "select au_fname, au_lname, city from authors where city=" will be executed for each value returned by "select city from authors where postalcode like "946%"". Is this right?
Thanks

Answer 1

Hi,
user13162080 wrote:
Hy, I have a query and a subquery(both of them are selects) and I want the outer query to be executed for each value returned by the subquery but the outer query has to return only one result. Sorry, I don't understand what you want. What if several rows in the table meet all the criteria? Do you only want some kind of summary of all of them?
Whenever you have a question, post a little sample data (CREATE TABLE and INSERT statements), and the results you want from that data.
For example, the following query is right for what I want?:
the following query finds out which authors live in the same city by looking at the postal code:
select au_fname, au_lname, city
from authors
where city = all
(select city
from authors
where postalcode like "946%")As I said earlier, I don't know what you want , but whatever it is, the query above is probably not the right way to get it.
Perhaps you meant "IN" instead of "= ALL", like this:
SELECT  au_fname, au_lname, city
FROM      authors
WHERE      city     IN          -- ***NOT***  = all
          (   SELECT  city
              FROM    authors
              WHERE   postalcode LIKE "946%"
I undertand the sentence "select au_fname, au_lname, city from authors where city=" will be executed for each value returned by "select city from authors where postalcode like "946%"". Is this right?No; the main query will be executed once, and the sub-query will be executed once. The rows in the main query will be compared to the results of the sub-query, to see if they will be included in the result set or not. This goes both for the original query you posted, and the modified version I posted.
InoL wrote:
Maybe I don't understand exactly what you want, or you gave a bad example. But your example is simply:
select au_fname, au_lname, city
from authors
where postalcode like '946%'
No; consider this data:
INSERT INTO authors (au_lname, city, postalcode) VALUES ('Virgil',     'Oakland',     '94601');
INSERT INTO authors (au_lname, city, postalcode) VALUES ('Steinbeck',     'Oakland',     NULL);
INSERT INTO authors (au_lname, city, postalcode) VALUES ('Grass',     'Emeryville',     '94608');What OP posted would return no rows, because nobody is in both 'Oakland' and 'Emeryville'.
What you posted would return 'Virgil' and 'Grass'.
What I posted would reutn all 3 rows, including Steinbeck, whose city is known to be related to a '946%' postalcode, even though his own postalcode is missing.
Edited by: Frank Kulash on Feb 9, 2011 1:06 PM