We are trying to determine which people have not yet turned in
evaluations for particular presentations

The evaluation table has the following structure
> describe evs;
+-----------------+---------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------------+---------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| prsn_id | int(11) | NO | | NULL | |
| prsntn_id | int(11) | NO | | NULL | |
+-----------------+---------+------+-----+---------+----------------+

the presentation table is as follows:

mysql> describe prsnttns;
+-----------+--------------+------+-----+---------+----------------+
| Field | Type | Null | Key | Default | Extra |
+-----------+--------------+------+-----+---------+----------------+
| id | int(11) | NO | PRI | NULL | auto_increment |
| ttl | varchar(255) | YES | | NULL | |
+-----------+--------------+------+-----+---------+----------------+

What we want to know is for each person (identified as prsn_id) which
evaluations have not yet been turned in. In this case, for a particular
presentation id (prsntn_id) we want to know if there exists a row in
the ev table for that person (prsn_id), indicating an evaluation was
entered. My understanding of joins is quite limited. I can determine
which presentations have NO evaluations via the following:

select evs.prsn_id,prsntns.id from prsntns left join evs on
evs.prsntn_id=prsntns.id where evs.prsn_id is null;

but that is not quite what I want. Any suggestions would be appreciated.

Tim

On 21 May, 05:54, <t...@tonto.stanford.edu> wrote:
> We are trying to determine which people have not yet turned in
> evaluations for particular presentations
>
> The evaluation table has the following structure> describe evs;
>
> +-----------------+---------+------+-----+---------+----------------+
> | Field | Type | Null | Key | Default | Extra |
> +-----------------+---------+------+-----+---------+----------------+
> | id | int(11) | NO | PRI | NULL | auto_increment |
> | prsn_id | int(11) | NO | | NULL | |
> | prsntn_id | int(11) | NO | | NULL | |
> +-----------------+---------+------+-----+---------+----------------+
>
> the presentation table is as follows:
>
> mysql> describe prsnttns;
> +-----------+--------------+------+-----+---------+----------------+
> | Field | Type | Null | Key | Default | Extra |
> +-----------+--------------+------+-----+---------+----------------+
> | id | int(11) | NO | PRI | NULL | auto_increment |
> | ttl | varchar(255) | YES | | NULL | |
> +-----------+--------------+------+-----+---------+----------------+
>
> What we want to know is for each person (identified as prsn_id) which
> evaluations have not yet been turned in. In this case, for a particular
> presentation id (prsntn_id) we want to know if there exists a row in
> the ev table for that person (prsn_id), indicating an evaluation was
> entered. My understanding of joins is quite limited. I can determine
> which presentations have NO evaluations via the following:
>
> select evs.prsn_id,prsntns.id from prsntns left join evs on
> evs.prsntn_id=prsntns.id where evs.prsn_id is null;
>
> but that is not quite what I want. Any suggestions would be appreciated.
>
> Tim

From your description, I do not see that a JOIN is required:
SELECT
*
FROM evs
WHERE prsntn_id = x and prsn_id = y