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Thread: sub queries question

  1. #1
    Join Date
    May 2010
    Posts
    166

    sub queries question

    I am trying to study sub queries, but correlated sub queries is not yet clear to me. Suppose I have users_tbl that have id and user_name column, I also have login_tbl that have id and last_login columns. How can I join the two table,getting the user_name and last_login using sub query? I know how to do this with join. Like this below

    SELECT u.user_username, d.date_time_in
    FROM users_tbl
    JOIN last_login d ON u.user_id = d.user_id

    But how to do this using sub query instead?
    Last edited by anishgiri; 01-13-2013 at 10:58 PM.

  2. #2
    Join Date
    Aug 2004
    Location
    Ankh-Morpork
    Posts
    22,335
    Is something like this what you are trying to do?
    Code:
    SELECT
      u.user_username,
      (
        SELECT date_time_in FROM last_login WHERE user_id = u.user_id
        ORDER BY date_time_in DESC LIMIT 1
      ) AS last_time_in
    FROM users_tbl AS u
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  3. #3
    Join Date
    May 2010
    Posts
    166
    I want a condition where only the user id that have record in last_login table will show the username and date_time_in record. So i tried this.


    SELECT u.user_username, (

    SELECT date_time_in
    FROM last_login
    WHERE user_id = u.user_id
    ORDER BY date_time_in DESC
    LIMIT 1
    ) AS last_time_in
    FROM users_tbl AS u
    WHERE u.user_id
    IN (

    SELECT user_id
    FROM last_login
    )
    Now this will display the user_name(with corresponding date_time_in) of the user once. But what i want is if the user has 3 last_login record the query will display his name 3 times also(with corresponding date_time_in) . How to do that with subquery? Also why does it produce the error Subquery returns more than 1 row when I removed ORDER BY date_time_in DESC
    LIMIT 1
    ?
    Last edited by anishgiri; 01-14-2013 at 03:48 AM.

  4. #4
    Join Date
    Aug 2004
    Location
    Ankh-Morpork
    Posts
    22,335
    For the last question, since I just wanted one value, without the limit 1 clause, you can get multiple results, which is invalid in that context.

    For what you are describing, I would not use a sub-query, just use an inner join.
    "Well done....Consciousness to sarcasm in five seconds!" ~ Terry Pratchett, Night Watch

    How to Ask Questions the Smart Way (not affiliated with this site, but well worth reading)

    My Blog
    cwrBlog: simple, no-database PHP blogging framework

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