将查询 LEFT JOIN 更改为 FULL OUTER JOIN（尊重空值） - 如何更改此设置（之后没有可怕的性能）？

Question

具有如下结构的实体属性值设置table（它来自第三方插件，我无法更改数据库设计）：

现在我想创建一个 table，其中 data_id 是 id，名称是列，值是它们的值。仍然每个 data_id 都没有每个名称的值，因此我希望结果中的值为 NULL（或空），以防原始 table 中没有此名称的值。

现在我已经编写了一个 PHP 脚本，它正在为我生成所需的查询：

$ihash = function($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);

这是生成的查询的样子：

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

结果table生成正确，但结果为空：

原因是没有所有列值的结果被过滤掉，但它们应该有空值（每个data_id有一个结果存在于 table）。我正在考虑将所有 LEFT JOIN 替换为 FULL OUTER JOIN（必须在 MySQL 中伪造），但这使事情变得过于复杂，而且可能已经很糟糕的表现会比这更糟糕。我该如何解决这个问题？

Answer 1

MySQL不支持FULL OUTER JOIN。相反，使用聚合：

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;

将查询 LEFT JOIN 更改为 FULL OUTER JOIN（尊重空值） - 如何更改此设置（之后没有可怕的性能）？

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

php

mysql

sql

join

entity-attribute-value