Joining two tables and show data from one if there is any - sql

Question

I have these two tables that i need to join
fields_data fields
+------------+-----------+------+ +------+-------------+----------+
| relationid | fieldname | data | | name | displayname | position |
+------------+-----------+------+ +------+-------------+----------+
| 2 | ftp | test | | user | Username | top |
| 2 | other | 1234 | | pass | Password | top |
+------------+-----------+------+ | ftp | FTP | top |
| log | Log | top |
| txt | Text | mid |
+------+-------------+----------+
I want to get all the rows from the "fields" table if they have the position "top" AND if a row has a match on name = fieldname from fields_data it should also show the data. This is my join
SELECT
fd.`data`,
fd.`relationid`,
fd.`fieldname`,
f.`name`,
f.`displayname`
FROM `fields` AS f
LEFT OUTER JOIN `fields_data` AS fd
ON fd.`fieldname` = f.`name`
WHERE f.`position`='top' AND (fd.`relationid`='3' OR fd.`relationid` IS NULL)
My problem is that the above query only gives me this result:
+------+------------+-----------+------+-------------+
| data | relationid | fieldname | name | displayname |
+------+------------+-----------+------+-------------+
| NULL | NULL | NULL | user | Username |
| NULL | NULL | NULL | pass | Password |
| NULL | NULL | NULL | log | Log |
+------+------------+-----------+------+-------------+
The field called "ftp" is missing due to it having a relation to "2".. However i still want to display it as result but like the others with NULL in it. And if the SQL query had "fd.relationid='2'" instead of 3 it would give same result, but with the row containing ftp in name, holding data in the three fields.
I hope you get what i mean.. My english is not the best.. Heres the result i want:
with above query containing fd.`relationid`='3'
+------+------------+-----------+------+-------------+
| data | relationid | fieldname | name | displayname |
+------+------------+-----------+------+-------------+
| NULL | NULL | NULL | user | Username |
| NULL | NULL | NULL | pass | Password |
| NULL | NULL | NULL | ftp | FTP |
| NULL | NULL | NULL | log | Log |
+------+------------+-----------+------+-------------+
with above query containing fd.`relationid`='2'
+------+------------+-----------+------+-------------+
| data | relationid | fieldname | name | displayname |
+------+------------+-----------+------+-------------+
| NULL | NULL | NULL | user | Username |
| NULL | NULL | NULL | pass | Password |
| test | 2 | ftp | ftp | FTP |
| NULL | NULL | NULL | log | Log |
+------+------------+-----------+------+-------------+

Answer 1

You want to move the condition to the on clause:
SELECT fd.`data`, fd.`relationid`, fd.`fieldname`, f.`name`, f.`displayname`
FROM `fields` f LEFT OUTER JOIN
`fields_data` fd
ON fd.`fieldname` = f.`name` AND fd.`relationid` = '3'
WHERE f.`position`='top' ;
It is interesting that the semantics of your query and this query are different -- and you found the exact situation: when there is a match on another value, the where clause form filters out the row. This will still keep everything.
As a note, the following also does what you want:
SELECT fd.`data`, fd.`relationid`, fd.`fieldname`, f.`name`, f.`displayname`
FROM `fields` f LEFT OUTER JOIN
(SELECT fd.*
FROM `fields_data` fd
WHERE fd.`relationid` = '3'
) fd
ON fd.`fieldname` = f.`name`
WHERE f.`position` = 'top' ;
I wouldn't recommend writing the query this way, particularly in MySQL (because the subquery is materialized). However, understanding why your version is different from these versions (and why these are the same) is a big step forward in mastering outer joins.