Left Join multiple times 2 tables using same column ON - php

Question

I have a table A where several columns values should be replaced from corresponding id which are stored in table B as in this scheme:
TABLE_A
object_id desc_id_1 desc_id_2 desc_id_3 desc_id_4
zxy_34 9999 9567 5555 5556
TABLE_B
desc_id complete_desc
1234 ABCD
9999 ZYXW
9567 POTG
5555 YUNR
5556 LPVR
……..
……..
Final show than should be like that:
object comp_desc_1 comp_desc_2 comp_desc_3 comp_desc_4
zxy_34 ZYXW POTG YUNR LPVR
Is it possible?
At the moment tried something like that:
$sql="SELECT table_a.object_id, table_b.complete_desc
FROM table_a
LEFT JOIN table_b ON table_a.desc_id_1 =table_b.complete_desc";
It works ok using output this way:
echo "<table border='1'>
<tr>
<th>object</th>
<th>comp_desc_1 </th>
</tr>";
while($row = mysqli_fetch_array($result)) {
echo "<tr>";
echo "<td>" . $row['object_id '] . "</td>";
echo "<td>" . $row['desc_id_1 '] . "</td>";
echo "</tr>";
}
echo "</table>";
The problem is that don’t really know how to go further.
Adding another left join like that stop the script and make load a blank page:
$sql="SELECT table_a.object_id, table_b.complete_desc, table_b.complete_desc
FROM table_a
LEFT JOIN table_b ON table_a.desc_id_1 =table_b.complete_desc
LEFT JOIN table_b ON table_a.desc_id_2 =table_b.complete_desc ";
…..
echo "<table border='1'>
<tr>
<th>object</th>
<th>comp_desc_1 </th>
<th>comp_desc_2 </th>
</tr>";
while($row = mysqli_fetch_array($result)) {
echo "<tr>";
echo "<td>" . $row['object_id '] . "</td>";
echo "<td>" . $row['desc_id_1 '] . "</td>";
echo "<td>" . $row['desc_id_2'] . "</td>";
echo "</tr>";
}
echo "</table>";
Some one can help me?

Answer 1

You need to give each instance of table_b in your query an alias, otherwise it can't be known which one you are referring to. You also need to give the separate complete_desc values different names so they don't overwrite each other in the output array. Try something like this:
$sql="SELECT table_a.object_id, b1.complete_desc AS desc_id_1, b2.complete_desc AS desc_id_2
FROM table_a
LEFT JOIN table_b b1 ON table_a.desc_id_1 = b1.complete_desc
LEFT JOIN table_b b2 ON table_a.desc_id_2 = b2.complete_desc ";
You also need to remove the spaces in the array keys in these lines:
echo "<td>" . $row['object_id '] . "</td>";
echo "<td>" . $row['desc_id_1 '] . "</td>";
they should be:
echo "<td>" . $row['object_id'] . "</td>";
echo "<td>" . $row['desc_id_1'] . "</td>";

Answer 2

It is important that I urge you to restructure your "table_a" table. It just isn't set up to run clean, efficient, concise queries.
To normalize the structure, I recommend these columns:
object_id
desc_id
desc_order
Then the "pivot" query that you desire can be executed with just one LEFT JOIN.
SELECT object_id,
MAX(CASE WHEN desc_order = 1 THEN b.desc_id ELSE NULL END) AS desc_id1,
MAX(CASE WHEN desc_order = 2 THEN b.desc_id ELSE NULL END) AS desc_id2,
MAX(CASE WHEN desc_order = 3 THEN b.desc_id ELSE NULL END) AS desc_id3,
MAX(CASE WHEN desc_order = 4 THEN b.desc_id ELSE NULL END) AS desc_id4
FROM `table_a` a
LEFT JOIN `table_b` b ON a.desc_id = b.desc_id
GROUP BY `object_id`
SQLFIDDLE DEMO
I can only hope that you aren't too far in your project to make this adjustment, because it will set you up for a much easier time developing in the future. (Plus flexibility is improved -- if you decide you want more than 4 columns, you can seamlessly include 5 or more columns in the result set.

Answer 3

To get what you want, the JOINs need to be on the matching IDs, then get the respective description AFTER the join. If a given "ID" may not be present, you would need a LEFT-JOIN meaning all records from the LEFT side (first table/alias) regardless of a match on the right (second table/alias). Also, good to get into using aliases to simplify readability, especially on long table names, or when you need the same table multiple times such as this case...
select
a.Object_ID,
id1.complete_desc ID1Descript,
id2.complete_desc ID2Descript,
id3.complete_desc ID3Descript,
id4.complete_desc ID4Descript
from
Table_A a
LEFT JOIN Table_B id1
on a.desc_id_1 = id1.desc_id
LEFT JOIN Table_B id2
on a.desc_id_2 = id2.desc_id
LEFT JOIN Table_B id3
on a.desc_id_3 = id3.desc_id
LEFT JOIN Table_B id4
on a.desc_id_4 = id4.desc_id
Your join would never have a match such as ID Number compared to a description. You need the ID from the columns in your Table_A joined to the ID column of TableB, hence multiple "alias" references for alias "id1", "id2", "id3", "id4"...
BTW, may people frown on too many underscores for tables and/or columns. Get used to CamelCaseColumnNaming vs Using_Underscore_For_Columns. Just a suggestion...