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Asked: 2026-05-31T12:18:42+00:00

I am trying to get data from multiple tables at once. In my old

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I am trying to get data from multiple tables at once. In my old system I would make a query, get the data I needed, and then use it for the next query. I wanted to avoid making multiple queries so I combined them with joins:

 SELECT `to`.`id` , `to`.`group`, `to`.`email`, `pTo`.`min_offer`, COUNT(`offerRes`.`id`), `settingsRes`.`email`
                                FROM `members` AS `to`

                                LEFT JOIN (
                                  SELECT `id` , `min_offer`
                                  FROM `profile`
                                ) pTo ON pTo.id = to.id

                                LEFT JOIN (
                                  SELECT `id`, `from`,`to`
                                  FROM `offers`
                                ) offerRes ON offerRes.from = ? && offerRes.to = to.id

                                LEFT JOIN (
                                  SELECT `id`, `email`
                                  FROM `user_settings`
                                )  settingsRes ON settingsRes.id = to.id

                                WHERE to.id = ? LIMIT 1

I use the ID that is passed in (to.id) to obtain results from the profiles table and the offers table and the user settings table. These tables all have a PRIMARY id to which everything is mapped to. My PHP users prepared statements however I replaced the ? with hard values in PHPMyAdmin. My EXPLAIN is as follows:

id  select_type     table   type    possible_keys   key     key_len     ref     rows    Extra
1   PRIMARY         to     const    PRIMARY       PRIMARY   4         const     1   
1   PRIMARY     <derived2>  ALL     NULL            NULL    NULL    NULL        45  
1   PRIMARY     <derived3>  ALL     NULL            NULL    NULL    NULL        1   
1   PRIMARY     <derived4>  ALL     NULL            NULL    NULL    NULL        15  
4   DERIVED     user_settingsALL    NULL            NULL    NULL    NULL        15  
3   DERIVED     offers      system  NULL            NULL    NULL    NULL        1   
2   DERIVED     profile     ALL     NULL            NULL    NULL    NULL        45

Why is MySQL going through all 45 rows in the profile field when it can do a simple lookup as my query is based on the PRIMARY key id? Any way to make this more efficent? All I want is a table of the following:

id (from members) - this is optional as this is what is obtained from the user and passed in to SELECT against
group (from members) - corresponding to the id that is passed in
email (from members) - corresponding to the id that is passed in
min_offer (from profile) - also corresponding to the id that is passed in
NUM_ROWS(from offers) - number of rows corresponding to the id taht is passed in and another number that is also passed in.  The offers table has a PRIMARY id but it is a different ID than what is passed in.
email (from user_settings) - corresponding to the id that is passed in
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1 Answer

  1. Editorial Team

    Why is MySQL going through all 45 rows in the profile field when it can do a simple lookup as my query is based on the PRIMARY key id?

    Because you’re joining to the results of sub-queries, which will include every record from the table:

    LEFT JOIN (
  SELECT `id` , `min_offer`
  FROM `profile`
) pTo ON pTo.id = to.id

-- etc.

    Instead, join to the tables directly:

    LEFT JOIN `profile` AS pTo ON pTo.id = to.id

LEFT JOIN `offers` AS offerRes ON offerRes.from = ? && offerRes.to = to.id

LEFT JOIN `user_settings` AS settingsRes ON settingsRes.id = to.id
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