Query by Value in Adjacent Column in MySQL - mysql

Question

I believe all questions of this nature start out this way...
I have two tables: fields and data. Fields describes the column names of a (non-existant) table and data contains the data of that (non-existant) table. Like so...
Fields:
+----+---------+-------------+---------------+-------+----------+
| ID | F_ORDER | NAME | LABEL | VALUE | TYPE |
+----+---------+-------------+---------------+-------+----------+
| 1 | 1 | IS_EMPLOYEE | Region | | checkbox |
| 2 | 3 | EM_AVATAR | Avatar | | avatar |
| 3 | 4 | EM_JOBTITLE | Job Title | | text |
| 4 | 5 | EM_COMPANY | Company | | text |
| 5 | 6 | EM_PHONE | Phone | | text |
| 6 | 2 | EM_ORDER | Display Order | 5 | text |
+----+---------+-------------+---------------+-------+----------+
Data:
+-----+----------+---------+--------------------------------------+
| ID | FIELD_ID | USER_ID | VALUE |
+-----+----------+---------+--------------------------------------+
| 5 | 1 | 1 | YES |
| 6 | 2 | 1 | |
| 7 | 3 | 1 | Owner |
| 8 | 4 | 1 | Acme, Inc. |
| 9 | 5 | 1 | 123-456-7987 |
| 150 | 5 | 31 | 123-623-5555 |
| 149 | 4 | 31 | Acme, Inc. |
| 148 | 3 | 31 | Sales and Customer Support |
| 147 | 2 | 31 | |
| 146 | 1 | 31 | YES |
| 26 | 1 | 6 | NO |
| 27 | 2 | 6 | http://example.com/avi/avi.jpeg |
| 28 | 3 | 6 | CEO |
| 29 | 4 | 6 | Acme |
| 30 | 5 | 6 | [phone] |
| 31 | 1 | 7 | NO |
| 32 | 2 | 7 | http://example.com/avi/avi.jpeg |
| 33 | 3 | 7 | VP, Services |
| 34 | 4 | 7 | Acme |
| 35 | 5 | 7 | [phone] |
| 36 | 1 | 14 | NO |
| 37 | 2 | 14 | http://example.com/avi/avi.jpeg |
| 38 | 3 | 14 | Senior Accountant |
| 39 | 4 | 14 | Acme |
| 40 | 5 | 14 | [phone] |
| 41 | 1 | 10 | NO |
| 42 | 2 | 10 | http://example.com/avi/avi.jpeg |
| 43 | 3 | 10 | President |
| 44 | 4 | 10 | Acme |
| 45 | 5 | 10 | [phone] |
| 46 | 1 | 12 | NO |
| 47 | 2 | 12 | http://example.com/avi/avi.jpeg |
| 48 | 3 | 12 | Services Supervisor |
| 49 | 4 | 12 | Acme |
| 50 | 5 | 12 | [phone] |
| 51 | 1 | 11 | NO |
| 52 | 2 | 11 | http://example.com/avi/avi.jpeg |
| 53 | 3 | 11 | Operations Supervisor |
| 54 | 4 | 11 | Acme |
| 55 | 5 | 11 | [phone] |
| 56 | 1 | 8 | NO |
| 57 | 2 | 8 | http://example.com/avi/avi.jpeg |
| 58 | 3 | 8 | General Information |
| 59 | 4 | 8 | Acme |
| 60 | 5 | 8 | [phone] |
| 61 | 1 | 9 | NO |
| 62 | 2 | 9 | http://example.com/avi/avi.jpeg |
| 63 | 3 | 9 | VP, Sales |
| 64 | 4 | 9 | Acme |
| 65 | 5 | 9 | [phone] |
+-----+----------+---------+--------------------------------------+
The basic verbiage of the query I'm looking for is: I want all information for all people who are employees (IS_EMPLOYEE = "YES") and ordered by their display order column (EM_ORDER).
My query so far gets me nowhere. I get results a little like this:
+--------+------------+------------+-------+---------+-------+
ID FIELD_ID NAME LABEL TYPE VALUE
+--------+------------+------------+-------+---------+-------+
7 1 IS_EMPLOYEE Region checkbox YES
+--------+------------+------------+-------+---------+-------+
What I need are results like this:
+-------+------------+---------+-----------+----------+-------------+--------+
USER_ID IS_EMPLOYEE EM_AVATAR EM_JOBTITLE EM_COMPANY EM_PHONE EM_ORDER
+-------+------------+---------+-----------+----------+-------------+--------+
6 YES http:// CEO Acme 123-123-555 5
+-------+------------+---------+-----------+----------+-------------+--------+
And of course I'm trying to get it all back into PHP as a usable array ($results['user_id']['jobtitle'] etc.). I could just get everything and work through it with PHP, but I'm trying to learn MySQL and I think this is a faster method than doing several foreach blocks ... although I could be wrong.
Thanks in advance for any help.

Answer 1

Your data table is organized in the Entity-Attribute-Value manner, which is not a valid way of storing relational data. It's no wonder that it's awkward to retrieve it with SQL into a conventional row with one column per attribute.
To do this with SQL, you basically need to do a pivot query. This is hard to write and terribly inefficient to execute.
You're better off fetching all the data for employees:
SELECT e.*, f.label
FROM Data AS is_employee
JOIN Data AS e USING (user_id)
JOIN Fields AS f ON e.field_id = f.id
WHERE (is_employee.field_id, is_employee.value) = (1, 'Yes');
And then reorganizing it into a collection of associative arrays in PHP code:
while ($row = $stmt->fetch()) {
$data[ $row['user_id'] ][ $row['label'] ] = $row['value'];
}

Answer 2

This is basically a PIVOT but MySQL does not have a PIVOT function so you can replicate it using an aggregate function and a CASE statement.
A Static version is when you know all of the values that you want to transform into columns (these are your field names):
select
d.user_id,
max(case when f.name = 'IS_EMPLOYEE' then d.value else null end) IS_EMPLOYEE,
max(case when f.name = 'EM_AVATAR' then d.value else null end) EM_AVATAR,
max(case when f.name = 'EM_JOBTITLE' then d.value else null end) EM_JOBTITLE,
max(case when f.name = 'EM_COMPANY' then d.value else null end) EM_COMPANY,
max(case when f.name = 'EM_PHONE' then d.value else null end) EM_PHONE,
max(case when f.name = 'EM_ORDER' then d.value else null end) EM_ORDER
from data d
left join fields f
on f.id = d.FIELD_ID
group by d.user_id
See SQL Fiddle with Demo
Result:
| USER_ID | IS_EMPLOYEE | EM_AVATAR | EM_JOBTITLE | EM_COMPANY | EM_PHONE | EM_ORDER |
---------------------------------------------------------------------------------------------------------------------------------
| 1 | YES | (null) | Owner | Acme, Inc. | 123-456-7987 | (null) |
| 6 | NO | http://example.com/avi/avi.jpeg | CEO | Acme | [phone] | (null) |
| 7 | NO | http://example.com/avi/avi.jpeg | VP, Services | Acme | [phone] | (null) |
| 8 | NO | http://example.com/avi/avi.jpeg | General Information | Acme | [phone] | (null) |
| 9 | NO | http://example.com/avi/avi.jpeg | VP, Sales | Acme | [phone] | (null) |
| 10 | NO | http://example.com/avi/avi.jpeg | President | Acme | [phone] | (null) |
| 11 | NO | http://example.com/avi/avi.jpeg | Operations Supervisor | Acme | [phone] | (null) |
| 12 | NO | http://example.com/avi/avi.jpeg | Services Supervisor | Acme | [phone] | (null) |
| 14 | NO | http://example.com/avi/avi.jpeg | Senior Accountant | Acme | [phone] | (null) |
| 31 | YES | (null) | Sales and Customer Support | Acme, Inc. | 123-623-5555 | (null) |
If you have an unknown number of values to turn into columns, then you can use a prepared statement to generate the sql dynamically.
SET #sql = NULL;
SELECT
GROUP_CONCAT(DISTINCT
CONCAT(
'max(case when f.name = ''',
name,
''' then d.value end) AS ',
name
)
) INTO #sql
FROM fields;
SET #sql = CONCAT('SELECT d.user_id, ', #sql, '
from data d
left join fields f
on f.id = d.FIELD_ID
GROUP BY d.user_id');
PREPARE stmt FROM #sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;
See SQL Fiddle with Demo
Both will produce the same result.
You can then add a WHERE clause to filter out any of the unneeded rows.