Converting sub-queries to joins - mysql

Question

Given the following schema...
how would I go about converting the following query to use joins (to work with MySQL)?
SELECT submissions.SubmissionID,
(SELECT data.DataContent FROM data WHERE data.DataFieldName =
(SELECT forms.FormEmailFieldName FROM forms WHERE forms.FormID = submissions.SubmissionFormID)
AND data.DataSubmissionID = submissions.SubmissionID) AS EmailAddress,
(SELECT data.DataContent FROM data WHERE data.DataFieldName =
(SELECT forms.FormNameFieldName FROM forms WHERE forms.FormID = submissions.SubmissionFormID)
AND data.DataSubmissionID = submissions.SubmissionID) AS UserName
FROM submissions
WHERE submissions.SubmissionFormID = 4
Below is some sample data and my desired result...
+--------+--------------------+-------------------+
| forms | | |
+--------+--------------------+-------------------+
| FormID | FormEmailFieldName | FormNameFieldName |
| 4 | UserEmailAddress | UserName |
| 5 | email | name |
+--------+--------------------+-------------------+
+--------------+------------------+
| submissions | |
+--------------+------------------+
| SubmissionID | SubmissionFormID |
| 10 | 4 |
| 11 | 5 |
| 12 | 5 |
+--------------+------------------+
+--------+------------------+------------------+------------------+
| data | | | |
+--------+------------------+------------------+------------------+
| DataID | DataSubmissionID | DataFieldName | DataContent |
| 1 | 10 | UserEmailAddress | user#example.com |
| 2 | 10 | UserName | Paul D'Otherone |
| 3 | 11 | email | bob#bobs.com |
| 4 | 11 | name | Bob Bobbington |
| 5 | 11 | phone | 01234 5678910 |
| 6 | 11 | country | UK |
| 7 | 12 | name | Sheila Sausages |
| 8 | 12 | country | UK |
+--------+------------------+------------------+------------------+
+--------------------------+------------------+-----------------+
| DESIRED RESULT | | |
+--------------------------+------------------+-----------------+
| submissions.SubmissionID | EmailAddress | UserName |
| 10 | user#example.com | Paul D'Otherone |
| 11 | bob#bobs.com | Bob Bobbington |
| 12 | NULL | Sheila Sausages |
+--------------------------+------------------+-----------------+
Also see http://sqlfiddle.com/#!2/78dea/1/0
I've tried various combinations of inner joins and left joins but can't get a result set in the same format as the above query. I am still learning how to use joins and am finding this hard to get my head around.

Answer 1

You can do this with one join to forms and two to data, one for each field.
SELECT s.SubmissionID, de.DataContent as EmailAddress,
dn.DataContent as UserName
FROM submissions s LEFT JOIN
forms f
ON f.FormId = s.SubmissionFormID LEFT JOIN
data dn
ON d.DataFieldName = f.FormNameFieldName LEFT JOIN
data de
ON d.DataFieldName = f.FormEmailFieldName
WHERE s.SubmissionFormID = 4
In case there is missing data, then you want to use LEFT JOIN. This will ensure that you get all the rows (which your original query does).
I also added table aliases into the query. These make queries easier to write and to read.

Answer 2

Without having sample data, just a quick try:
SELECT
s.SubmissionID,
d1.DataContent AS EmailAddress,
d2.DataContent AS Username
FROM submissions s
JOIN forms AS f1 ON (f1.FormID = s.SubmissionFormID)
JOIN data AS d1 ON (d1.DataFieldName = f1.FormEmailFieldName)
JOIN data AS d2 ON (d2.DataFieldName = f1.FormNameFieldName)
WHERE s.SubmissionFormID = 4
sql fiddle for it: http://sqlfiddle.com/#!2/9f917/1

Answer 3

Starting from Olli's and Gordon's suggestions I have come up with the following which gives the same result set as my original query...
SELECT
s.SubmissionID,
d1.DataContent AS EmailAddress,
d2.DataContent AS Username
FROM submissions s
INNER JOIN forms AS f1 ON (f1.FormID = s.SubmissionFormID)
LEFT JOIN data AS d1 ON (d1.DataFieldName = f1.FormEmailFieldName and d1.DataSubmissionID = s.SubmissionID)
LEFT JOIN data AS d2 ON (d2.DataFieldName = f1.FormNameFieldName and d2.DataSubmissionID = s.SubmissionID)
WHERE s.SubmissionFormID = 4
Does this seem like the correct way to go about achieving what I want?