Change query LEFT JOIN to FULL OUTER JOIN (respecting null values) - how to change this (without horrible performance afterwards)?

Question

Having an Entity-Attribute Value setup table which is structured like this (it is from a third party plugin, I can not change the database design):

Now I want to create a table whith data_id being the id, the names being the columns and the values being their values. Still every data_id does not have a value for every name therefore I want the value in the result to be NULL (or empty) in case there is no value for this name within the original table.

Now I have written a PHP script which is generating the required query for me:

$ihash = function($len = 10){
        return substr(str_shuffle(str_repeat("abcdefghijklmnopqrstuvwxyz", 10)), 0, 10);
    };
$columns = $wpdb->get_col("SELECT DISTINCT name FROM ".$wpdb->prefix."cf7_vdata_entry");

$fromselects = [];
$left_joins = [];
$wheres = [];
$used=[];

foreach($columns as $column) {
    $m = $ihash();
    while(in_array($m,$used)) {
        $m = $ihash();
    }
    array_push($used,$m);
    array_push($fromselects,"$m.value as `$column`");
    $left_joins  .= " LEFT JOIN wp_cf7_vdata_entry AS $m ON a.data_id = $m.data_id ";
    array_push($wheres,"$m.name = '$column'");
}

$query = "SELECT a.data_id, ".implode(", ",$fromselects)."
  FROM (SELECT DISTINCT data_id FROM wp_cf7_vdata_entry) AS a JOIN
  ".$left_joins."
 WHERE ".implode(" AND ",$wheres);

This is how a generated query looks like:

SELECT a.data_id, 
       vtddnqrdjy.value AS `foerderung`, 
       fwfyxgczvn.value AS `company`, 
       jwlpmnbepe.value AS `firstname`,
       -- ... more fields
FROM   (SELECT DISTINCT data_id 
        FROM   wp_cf7_vdata_entry) AS a 
       JOIN wp_cf7_vdata_entry AS vtddnqrdjy 
         ON a.data_id = vtddnqrdjy.data_id 
       JOIN wp_cf7_vdata_entry AS fwfyxgczvn 
         ON a.data_id = fwfyxgczvn.data_id 
       JOIN wp_cf7_vdata_entry AS jwlpmnbepe 
         ON a.data_id = jwlpmnbepe.data_id 
       -- ... more joins
WHERE  vtddnqrdjy.name = 'foerderung' 
       AND fwfyxgczvn.name = 'company' 
       AND jwlpmnbepe.name = 'firstname' 
       AND mloxjygcqp.name = 'lastname' 
       -- ... more fields
LIMIT  10 

The result table is generated correctly, but the result is empty:

The reason is that results which do not have ALL column values are filtered out, but they should have null values instead (having exactly one result for each data_id existing in the table). I was thinking about replacing all LEFT JOIN s with FULL OUTER JOIN s (which has to be faked in MySQL), but this is overcomplicating things and the probably already bad performance will be extremly bad than. How could I solve this?

Answer 1

MySQL does not support FULL OUTER JOIN . Instead, use aggregation:

SELECT de.data_id,
       MAX(CASE WHEN de.name = 'foerderung' THEN de.value END) as foerderung, 
       MAX(CASE WHEN de.name = 'company' THEN de.value END) as company
           . . .   -- just repeat for each column
FROM wp_cf7_vdata_entry de
GROUP BY de.data_id;