Question

Need some help with a query. I have 2 tables in a database, contacts & tasks.

mysql> describe contacts;
+------------+--------------+------+-----+---------+----------------+
| Field      | Type         | Null | Key | Default | Extra          |
+------------+--------------+------+-----+---------+----------------+
| contact_id | int(11)      | NO   | PRI | NULL    | auto_increment |
| last_name  | varchar(100) | YES  |     | NULL    |                |
| first_name | varchar(100) | YES  |     | NULL    |                |
| email      | varchar(50)  | YES  |     | NULL    |                |
| phone      | varchar(20)  | YES  |     | NULL    |                |
| school_id  | varchar(12)  | NO   |     | NULL    |                |
| access     | char(1)      | YES  |     | NULL    |                |
+------------+--------------+------+-----+---------+----------------+

mysql> describe tasks;
+-------------------+--------------+------+-----+---------+----------------+
| Field             | Type         | Null | Key | Default | Extra          |
+-------------------+--------------+------+-----+---------+----------------+
| task_id           | int(11)      | NO   | PRI | NULL    | auto_increment |
| task_name         | varchar(125) | YES  |     | NULL    |                |
| task_date         | date         | YES  |     | NULL    |                |
| task_description  | text         | YES  |     | NULL    |                |
| contact_id1       | int(11)      | YES  |     | NULL    |                |
| contact_id2       | int(11)      | YES  |     | NULL    |                |
| contact_id3       | int(11)      | YES  |     | NULL    |                |
| contact_id4       | int(11)      | YES  |     | NULL    |                |
| contact_id5       | int(11)      | YES  |     | NULL    |                |
| contact_id6       | int(11)      | YES  |     | NULL    |                |
| completed         | char(1)      | YES  |     | NULL    |                |
+-------------------+--------------+------+-----+---------+----------------+

I want to get the info from the tasks table but I want to replace the contact_id1-contact_id6 with some of the fields from the matching contact_id. I have been messing around with nested select statements which will work but is very messy. It just seems there has to be a cleaner way to do this. I thought I had some with this,

SELECT tasks.task_id, tasks.task_name, tasks.reminder_time, tasks.reminder_interval, CONCAT (contact_1.first_name, " ", contact_1.last_name) as contact_1_name, CONCAT(contact_2.first_name, contact_2.last_name) as contact_2_name, CONCAT(contact_3.first_name, contact_3.last_name) as contact_3_name
FROM tasks
JOIN contacts contact_1 ON tasks.contact_id1 = contact_1.contact_id
JOIN contacts contact_2 ON tasks.contact_id2 = contact_2.contact_id
JOIN contacts contact_3 ON tasks.contact_id3 = contact_3.contact_id

But my issue here is that it is not showing tasks where any of the values of contact_id1-contact_id6 have a value of 0 which is the default value if no contact_id is set for it.

Any help you can provide would be great.

Answer 1

You can make a view that does normalize the tasks table then the join will be easy. You can make a permanent view or just for the select.

The view would need somehow to be a UNION

SELECT task_id, task_name, task_description, contact_id1 as contact_id, completed from tasks
UNION ALL
SELECT task_id, task_name, task_description, contact_id2              , completed from tasks
UNION ALL
SELECT task_id, task_name, task_description, contact_id3              , completed from tasks
UNION ALL
SELECT task_id, task_name, task_description, contact_id4              , completed from tasks
UNION ALL
SELECT task_id, task_name, task_description, contact_id5              , completed from tasks
UNION ALL
SELECT task_id, task_name, task_description, contact_id6              , completed from tasks

So then it looks like this

WITH tasks_easy as (    
    SELECT task_id, task_name, task_description, contact_id1 as contact_id, completed from tasks
    UNION
    SELECT task_id, task_name, task_description, contact_id2              , completed from tasks
    UNION
    SELECT task_id, task_name, task_description, contact_id3              , completed from tasks
    UNION
    SELECT task_id, task_name, task_description, contact_id4              , completed from tasks
    UNION
    SELECT task_id, task_name, task_description, contact_id5              , completed from tasks
    UNION
    SELECT task_id, task_name, task_description, contact_id6              , completed from tasks
 )
 select *
 from   tasks_easy t
 join   contacts c   on (t.contact_id = c.contact_id)

Have not tested this, but should work like this.

Edit: Actually was thinking about it. UNION ALL, at least for the temporary view is not necessary. Probably will bring you some time improvement if your tables are real large.