Php MySQL-将表结果转换为行

我有一张这种格式的桌子

id1 | id2 | id3 | id4  | id5
a   | b   | c   | null | null
a   | b   | d   | null | null
a   | b   |null | null | null

我需要a，b，c，d的结果

我用php做的，但是代码太难了

    <?php
    $sql="SELECT t1.id as id1, t2.id AS id2, t3.id AS id3, t4.id AS id4, t5.id AS id5
    FROM store_subcategory AS t1
    LEFT JOIN store_subcategory AS t2 ON t2.parent_id = t1.id
    LEFT JOIN store_subcategory AS t3 ON t3.parent_id = t2.id
    LEFT JOIN store_subcategory AS t4 ON t4.parent_id = t3.id
    LEFT JOIN store_subcategory AS t5 ON t5.parent_id = t4.id
    WHERE (t1.root_id=$cat or t1.id=$cat) and t1.store_id=$id";

    $result=$mysqli->query($sql);
    $subid=array();
    while($row=$result->fetch_assoc())
    {
        if($row["id1"]!=null) $subid[$row["id1"]]=1;
        if($row["id2"]!=null) $subid[$row["id2"]]=1;
        if($row["id3"]!=null) $subid[$row["id3"]]=1;
        if($row["id4"]!=null) $subid[$row["id4"]]=1;
        if($row["id5"]!=null) $subid[$row["id5"]]=1;
    }
    $subsarray=array();
    foreach($subid as $key => $value)
    {
        array_push($subsarray, $key);
    }
    $subid=implode(",", $subsarray);
    ?>

编辑2： 解决办法可能是这样

SELECT * FROM (
SELECT DISTINCT(t1.id)
    FROM store_subcategory AS t1
    WHERE (t1.root_id=$cat or t1.id=$cat) and t1.store_id=$id
UNION
SELECT DISTINCT(t2.id)
    FROM store_subcategory AS t1
    LEFT JOIN store_subcategory AS t2 ON t2.parent_id = t1.id
    WHERE (t1.root_id=$cat or t1.id=$cat) and t1.store_id=$id
UNION
SELECT DISTINCT(t3.id)
    FROM store_subcategory AS t1
    LEFT JOIN store_subcategory AS t2 ON t2.parent_id = t1.id
    LEFT JOIN store_subcategory AS t3 ON t3.parent_id = t2.id
    WHERE (t1.root_id=$cat or t1.id=$cat) and t1.store_id=$id
UNION
SELECT DISTINCT(t4.id)
    FROM store_subcategory AS t1
    LEFT JOIN store_subcategory AS t2 ON t2.parent_id = t1.id
    LEFT JOIN store_subcategory AS t3 ON t3.parent_id = t2.id
    LEFT JOIN store_subcategory AS t4 ON t4.parent_id = t3.id
    WHERE (t1.root_id=$cat or t1.id=$cat) and t1.store_id=$id
UNION
SELECT DISTINCT(t5.id)
    FROM store_subcategory AS t1
    LEFT JOIN store_subcategory AS t2 ON t2.parent_id = t1.id
    LEFT JOIN store_subcategory AS t3 ON t3.parent_id = t2.id
    LEFT JOIN store_subcategory AS t4 ON t4.parent_id = t3.id
    LEFT JOIN store_subcategory AS t5 ON t5.parent_id = t4.id
    WHERE (t1.root_id=$cat or t1.id=$cat) and t1.store_id=$id
) AS t_group WHERE id IS NOT NULL

---

我不知道该数据结构是如何有用的，但听起来您想要的是：

SELECT DISTINCT(ID) 
FROM 
    (select distinct(id1) AS ID
    from table
    UNION
    select distinct(id2) AS ID
    from table
    UNION
    select distinct(id3) AS ID
    from table)

SELECT DISTINCT(ID) 
FROM 
    (select distinct(id1) AS ID
    from table
    UNION
    select distinct(id2) AS ID
    from table
    UNION
    select distinct(id3) AS ID
    from table)