Issue with UNION in MySQL - mysql

Question

I have two tables.
rp_format
+-----+--+--------------+
| fid | | recordformat |
+-----+--+--------------+
| 1 | | CD |
| 2 | | Vinyl |
| 3 | | DVD |
+-----+--+--------------+
rp_records
+----+--+--------+
| id | | format |
+----+--+--------+
| 1 | | 1 |
| 2 | | 2 |
| 3 | | 3 |
+----+--+--------+
What I would like to achieve is to display everything from "rp_format". But I would also like make a check to see if there is a "fid"-value found in "format".
Example that should be displayed on page like this:
fid recordformat
1 CD Remove this format
2 Vinyl Remove this format
3 DVD Remove this format
But let's say an "fid" value is found in "format" then I would like it to be displayed like this on page:
fid recordformat
1 CD Remove this format
2 Vinyl Can't remove this format
3 DVD Remove this format
"Remove this format / Can't remove this format" is text that will be displayed by checking if "fid" = "format" using PHP.
Here is my SQL query so far:
global $wpdb;
$rpdb = $wpdb->prefix . 'rp_format';
$rpdb2 = $wpdb->prefix . 'rp_records';
$sql = "
SELECT *
FROM $rpdb
LEFT OUTER JOIN $rpdb2 ON $rpdb.fid = $rpdb2.format
UNION
SELECT *
FROM $rpdb
RIGHT OUTER JOIN $rpdb2 ON $rpdb.fid = $rpdb2.format
WHERE $rpdb.fid IS NOT NULL
";
The issue I have with this query is that when "fid" is found in "format" (let's say it's found 10 times) every of these 10 values will be outputed also.
How can this be fixed?
Kind regards
Johan

Answer 1

If I understand correctly you want to display some message depending on if the data exists on rp_records or not and avoid multiple display.
Consider the following
mysql> select * from rp_format;
+------+--------------+
| fid | recordformat |
+------+--------------+
| 1 | CD |
| 2 | Vinyl |
| 3 | DVD |
| 4 | Test |
+------+--------------+
4 rows in set (0.00 sec)
mysql> select * from rp_records;
+------+--------+
| id | format |
+------+--------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 2 |
| 5 | 1 |
+------+--------+
So the query is
select
f.*,
case
when r.format is not null then 'Can\'t remove' else 'Remove this' end
as message
from rp_format f
left join rp_records r on r.format = f.fid
group by f.fid ;
+------+--------------+--------------+
| fid | recordformat | message |
+------+--------------+--------------+
| 1 | CD | Can't remove |
| 2 | Vinyl | Can't remove |
| 3 | DVD | Can't remove |
| 4 | Test | Remove this |
+------+--------------+--------------+

Answer 2

Not sure that i correctly understand your logic with found and not found format, if i wrong - add to if condition r.format IS NOT NULL instead r.format IS NULL. And i think you no need to use union, you should use join:
SELECT
r.fid,
f.recordformat,
IF(r.format IS NULL, "Can't remove this format", "Remove this format")
FROM rp_format f
LEFT JOIN rp_records r ON f.fid = r.format
GROUP BY f.fid
;
I'm sure that something like this will help you!