MySQL右连接查询不返回任何值

Question

I am trying to return the top 10 'brands' which share a 'transaction id' with the selected brand ($brand). I began with a basic query which queries 1 table with the columns id and brand.

I am now wanting to get this to work on 2 separate tables(one for transactions and products respectively), with a common field (sku).

I began with this query, to get the initial logic (credit to a stackoverflow contributor), and it works perfectly.

$brand = $_POST['brand'];
$query = $db->prepare("SELECT brand
FROM transactions
WHERE
id IN (SELECT id FROM transactions WHERE brand = :brand)
AND brand <> :brand
GROUP BY brand
ORDER BY COUNT(*) DESC
LIMIT 10");
$query->bindparam(":brand", $brand);
$query->execute();

The above works great as I mentioned. And I modified it slightly below to incorporate related tables. BUT, it doesnt work. and returns no values.

Any takers. Much appreciated - Adam

$brand = $_POST['brand'];
$query = $db->prepare("SELECT `2_products`.`brand` 
FROM `2_products` 
RIGHT JOIN `1_txns` 
ON `2_products`.`sku`=`1_txns`.`sku` 
WHERE 
`1_txns`.`txn_id` IN (SELECT `1_txns`.`txn_id` FROM `1_txns` WHERE `2_products`.`brand` = :brand) 
AND `2_products`.`brand` <> :brand 
GROUP BY `2_products`.`brand` 
ORDER BY COUNT(*) DESC 
LIMIT 10");
$query->bindparam(":brand", $brand);
$query->execute();

I understand that there may be other methods to this such as create temp table, with just txn_id and brand columns, but again I'm unsure of how to do this.

edit attempt #2 - Here as per advice below I have moved the AND 2_products . brand <> :brand to immediately after the ON clause. Still 0 rows returned.

    $brand = $_POST['brand'];
$query = $db->prepare("SELECT `2_products`.`brand` 
FROM `2_products` 
RIGHT JOIN `1_txns` 
ON `2_products`.`sku`=`1_txns`.`sku` 
AND `2_products`.`brand` <> :brand
WHERE `1_txns`.`txn_id` IN (SELECT `1_txns`.`txn_id` FROM `1_txns` WHERE `2_products`.`brand` = :brand) 
GROUP BY `2_products`.`brand` 
ORDER BY COUNT(*) DESC 
LIMIT 10");
$query->bindparam(":brand", $brand);
$query->execute();

EDIT 3: Also taking into consideration the fact that in my inner select query the where clause is a column which is not selected from the inner query. and so I tried this: To which I got an error 'Operand should only contain 1 column(s)'

    SELECT `2_products`.`brand` 
FROM `2_products` 
RIGHT JOIN `1_txns` 
ON `2_products`.`sku`=`1_txns`.`sku` 
AND `2_products`.`brand` <> :brand 
WHERE `1_txns`.`txn_id` IN (SELECT `1_txns`.`txn_id`, `2_products`.`brand` 
                            FROM `1_txns` 
                            LEFT JOIN `2_products` 
                            ON `1_txns`.`sku`=`2_products`.`sku` 
                            WHERE `2_products`.`brand` = :brand) 
GROUP BY `2_products`.`brand` 
ORDER BY COUNT(*) DESC 
LIMIT 10;

Answer 1

When a LEFT / RIGHT join return no records, first check if WHERE has any filters referring to the table on the "weak" side. By "weak" side I mean the right table on a LEFT JOIN , or the left table on a RIGHT JOIN .

In your case, the left table is 2_products , so it might generate NULL fields, but in WHERE you have 2_products.brand <> :brand . If this condition fails, the record will be removed.

To avoid this, try moving this condition to the ON clause, after 2_products.sku=1_txns.sku , because filtering there will not remove the record if the condition fails. It should look this way:

ON 2_products.sku=1_txns.sku AND 2_products.brand <> :brand

And the WHERE clause should look like this:

WHERE 1_txns.txn_id IN (SELECT 1_txns.txn_id FROM 1_txns WHERE 2_products.brand = :brand)

Answer 2

This was solved in the following question on dba.stackexchange

DBA Question