Обсуждение: sub-query question

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sub-query question

От
Scott Frankel
Дата:
How does one embed a sub-query lookup to one table in order to
replace a foreign key id number with it's name in a SELECT on a
second table?

i.e.:  given the following two tables, I want to replace the color_id
of 1
with the color_name 'red.'  (The SQL to create the two tables follows
below.)

test=# SELECT * from users ;
  color_id | name |  the_date
----------+------+------------
         1 | john | 2004-03-10
         3 | jane | 2004-04-12
         1 | joe  | 2004-05-14
         2 | jepe | 2004-06-16
(4 rows)

test=# SELECT * from colors;
  color_id | color_name
----------+------------
         1 | red
         2 | green
         3 | blue
(3 rows)


My attempts yield an 'f' which looks suspiciously like a boolean false.
Is there an ordering issue with my sub-query, such that the sub-query
doesn't have enough info to perform its lookup?

Here's my query:

SELECT (
     u.color_id = (
         SELECT c.color_name
            FROM colors c
                WHERE color_id = 1)) AS color_name,
    u.name, u.the_date
    FROM users u
         WHERE u.color_id = 1
         ORDER BY u.the_date DESC LIMIT 1;

It returns:

  color_name | name |  the_date
------------+------+------------
  f          | joe  | 2004-05-14
(1 row)


Thanks!
Scott


Here's the SQL to create my test tables:

CREATE TABLE colors (color_id SERIAL PRIMARY KEY, color_name text);
CREATE TABLE users  (color_id integer REFERENCES colors, name text,
the_date date);

INSERT INTO colors  (color_name) VALUES ('red');
INSERT INTO colors  (color_name) VALUES ('green');
INSERT INTO colors  (color_name) VALUES ('blue');

INSERT INTO users   (color_id, name, the_date) VALUES (1, 'john',
'2004-03-10');
INSERT INTO users   (color_id, name, the_date) VALUES (3, 'jane',
'2004-04-12');
INSERT INTO users   (color_id, name, the_date) VALUES (1, 'joe',
'2004-05-14');
INSERT INTO users   (color_id, name, the_date) VALUES (2, 'jepe',
'2004-06-16');

Re: sub-query question

От
Franco Bruno Borghesi
Дата:
something == otherthing is a boolean expression, you are asking the
database to compare both values, u.color_id is not equal c.color_name,
that's why you get 'f'.

I guess that you want to replace the color_id from users by the
corresponding color_name from colors:

SELECT
    c.color_name, u.name, u.the_date
FROM
    users u
    INNER JOIN colors c ON (u.color_id=c.color_id)
WHERE
    u.color_id = 1
ORDER BY u.the_date DESC LIMIT 1;


If you were trying to do another thing, I'm sorry, I didn't get it :(

Scott Frankel wrote:

>
> How does one embed a sub-query lookup to one table in order to
> replace a foreign key id number with it's name in a SELECT on a
> second table?
>
> i.e.:  given the following two tables, I want to replace the color_id
> of 1
> with the color_name 'red.'  (The SQL to create the two tables follows
> below.)
>
> test=# SELECT * from users ;
>  color_id | name |  the_date
> ----------+------+------------
>         1 | john | 2004-03-10
>         3 | jane | 2004-04-12
>         1 | joe  | 2004-05-14
>         2 | jepe | 2004-06-16
> (4 rows)
>
> test=# SELECT * from colors;
>  color_id | color_name
> ----------+------------
>         1 | red
>         2 | green
>         3 | blue
> (3 rows)
>
>
> My attempts yield an 'f' which looks suspiciously like a boolean false.
> Is there an ordering issue with my sub-query, such that the sub-query
> doesn't have enough info to perform its lookup?
>
> Here's my query:
>
> SELECT (
>     u.color_id = (
>         SELECT c.color_name
>             FROM colors c
>                 WHERE color_id = 1)) AS color_name,
>     u.name, u.the_date
>     FROM users u
>          WHERE u.color_id = 1
>          ORDER BY u.the_date DESC LIMIT 1;
>
> It returns:
>
>  color_name | name |  the_date
> ------------+------+------------
>  f          | joe  | 2004-05-14
> (1 row)
>
>
> Thanks!
> Scott
>
>
> Here's the SQL to create my test tables:
>
> CREATE TABLE colors (color_id SERIAL PRIMARY KEY, color_name text);
> CREATE TABLE users  (color_id integer REFERENCES colors, name text,
> the_date date);
>
> INSERT INTO colors  (color_name) VALUES ('red');
> INSERT INTO colors  (color_name) VALUES ('green');
> INSERT INTO colors  (color_name) VALUES ('blue');
>
> INSERT INTO users   (color_id, name, the_date) VALUES (1, 'john',
> '2004-03-10');
> INSERT INTO users   (color_id, name, the_date) VALUES (3, 'jane',
> '2004-04-12');
> INSERT INTO users   (color_id, name, the_date) VALUES (1, 'joe',
> '2004-05-14');
> INSERT INTO users   (color_id, name, the_date) VALUES (2, 'jepe',
> '2004-06-16');
>
>
>
>
>
> ---------------------------(end of broadcast)---------------------------
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>
>               http://archives.postgresql.org
>

Re: sub-query question

От
Tom Lane
Дата:
Scott Frankel <leknarf@pacbell.net> writes:
> Here's my query:

> SELECT (
>      u.color_id = (
>          SELECT c.color_name
>             FROM colors c
>                 WHERE color_id = 1)) AS color_name,
>     u.name, u.the_date
>     FROM users u
>          WHERE u.color_id = 1
>          ORDER BY u.the_date DESC LIMIT 1;

I think you want

SELECT
     (SELECT c.color_name
        FROM colors c
        WHERE color_id = u.color_id) AS color_name,
     u.name, u.the_date
  FROM users u
  WHERE u.color_id = 1
  ORDER BY u.the_date DESC LIMIT 1;

The sub-select can refer to a variable of the outer query, as long
as you are careful to qualify it so it can't be mistaken for a variable
of the sub-select itself.

You could also express this query as a join.  If you are pulling a whole
lot of users rows, the join way would probably be more efficient.

            regards, tom lane

Re: sub-query question

От
Michael Fuhr
Дата:
On Fri, Nov 12, 2004 at 09:52:09AM -0800, Scott Frankel wrote:
>
> How does one embed a sub-query lookup to one table in order to
> replace a foreign key id number with it's name in a SELECT on a
> second table?

You're talking about joins.

http://www.postgresql.org/docs/7.4/static/tutorial-join.html
http://www.postgresql.org/docs/7.4/static/queries-table-expressions.html

> i.e.:  given the following two tables, I want to replace the color_id
> of 1
> with the color_name 'red.'  (The SQL to create the two tables follows
> below.)
>
> test=# SELECT * from users ;
>  color_id | name |  the_date
> ----------+------+------------
>         1 | john | 2004-03-10
>         3 | jane | 2004-04-12
>         1 | joe  | 2004-05-14
>         2 | jepe | 2004-06-16
>
> test=# SELECT * from colors;
>  color_id | color_name
> ----------+------------
>         1 | red
>         2 | green
>         3 | blue

There are at least four ways to write the join query you want:

SELECT color_name, name, the_date
FROM users NATURAL JOIN colors;

SELECT color_name, name, the_date
FROM users JOIN colors USING (color_id);

SELECT color_name, name, the_date
FROM users JOIN colors ON colors.color_id = users.color_id;

SELECT color_name, name, the_date
FROM users, colors
WHERE users.color_id = colors.color_id;

Debate exists about which of the above is "better."  I tend to use
the second and third forms because I think they most clearly document
how the tables are joined, and because I think of the WHERE clause
as meaning "...and here are the records I want from all that."

Of course that's just my opinion.  I could be wrong.

Re: sub-query question

От
Michael Fuhr
Дата:
On Fri, Nov 12, 2004 at 11:26:14AM -0700, Michael Fuhr wrote:

> There are at least four ways to write the join query you want:

I may have misunderstood what results you're looking for, but the
examples I gave may nevertheless be useful.  Sorry if they cause
any confusion.

--
Michael Fuhr
http://www.fuhr.org/~mfuhr/

Re: sub-query question

От
Scott Frankel
Дата:
Cooking with gas once again ;)  Thanks for the info on JOINs!
Scott




On Nov 12, 2004, at 9:52 AM, Scott Frankel wrote:

>
> How does one embed a sub-query lookup to one table in order to
> replace a foreign key id number with it's name in a SELECT on a
> second table?
>
> i.e.:  given the following two tables, I want to replace the color_id
> of 1
> with the color_name 'red.'  (The SQL to create the two tables follows
> below.)
>
> test=# SELECT * from users ;
>  color_id | name |  the_date
> ----------+------+------------
>         1 | john | 2004-03-10
>         3 | jane | 2004-04-12
>         1 | joe  | 2004-05-14
>         2 | jepe | 2004-06-16
> (4 rows)
>
> test=# SELECT * from colors;
>  color_id | color_name
> ----------+------------
>         1 | red
>         2 | green
>         3 | blue
> (3 rows)
>
>
> My attempts yield an 'f' which looks suspiciously like a boolean false.
> Is there an ordering issue with my sub-query, such that the sub-query
> doesn't have enough info to perform its lookup?
>
> Here's my query:
>
> SELECT (
>     u.color_id = (
>         SELECT c.color_name
>             FROM colors c
>                 WHERE color_id = 1)) AS color_name,
>     u.name, u.the_date
>     FROM users u
>          WHERE u.color_id = 1
>          ORDER BY u.the_date DESC LIMIT 1;
>
> It returns:
>
>  color_name | name |  the_date
> ------------+------+------------
>  f          | joe  | 2004-05-14
> (1 row)
>
>
> Thanks!
> Scott
>
>
> Here's the SQL to create my test tables:
>
> CREATE TABLE colors (color_id SERIAL PRIMARY KEY, color_name text);
> CREATE TABLE users  (color_id integer REFERENCES colors, name text,
> the_date date);
>
> INSERT INTO colors  (color_name) VALUES ('red');
> INSERT INTO colors  (color_name) VALUES ('green');
> INSERT INTO colors  (color_name) VALUES ('blue');
>
> INSERT INTO users   (color_id, name, the_date) VALUES (1, 'john',
> '2004-03-10');
> INSERT INTO users   (color_id, name, the_date) VALUES (3, 'jane',
> '2004-04-12');
> INSERT INTO users   (color_id, name, the_date) VALUES (1, 'joe',
> '2004-05-14');
> INSERT INTO users   (color_id, name, the_date) VALUES (2, 'jepe',
> '2004-06-16');
>
>
>
>
>
> ---------------------------(end of
> broadcast)---------------------------
> TIP 6: Have you searched our list archives?
>
>               http://archives.postgresql.org
>