SQL與多個值的左連接

Question

我有一個非常糟糕的表結構的舊數據庫，我試圖用更好的結構創建這些表。爲此，我需要匹配兩個表，以獲取類別的ID。

這裏是我的兩個舊錶：

表的categorys：

| ID | catname  | cat1 | cat2  | cat3 | cat4 | 
+----+--------------+--------+-------------+---------+------+ 
| 1 | bike  | bike | NULL  | NULL | NULL | 
| 2 | accessories | bike | accessories | NULL | NULL | 
| 3 | helmets  | bike | accessories | helmets | NULL | 
| 4 | lights  | bike | accessories | lights | NULL | 
| 5 | led   | bike | accessories | lights | led | 

表產品：我想擺脫列CAT1，2

| ID | productnr | productname | cat1 | cat2  | cat3 | cat4 | 
+----+-------------+---------------+-------+-------------+---------+------+ 
| 1 | 451157  | productya | bike | accessories | NULL | NULL | 
| 2 | 555523  | product11 | bike | accessories | helmets | NULL | 
| 3 | 234432  | helmetxqa | bike | accessories | helmets | NULL | 
| 4 | 666623  | lightblue | bike | accessories | lights | NULL | 
| 5 | 542123  | foobarlight | bike | accessories | lights | led | 

起初， 3和4來自產品表。

所以，我得到這樣一個結果：

| ID | catId | productnr | productname | 
+----+---------+------------+---------------+ 
| 1 | 2  | 451157  | productya | 
| 2 | 3  | 555523  | product11 | 
| 3 | 3  | 234432  | helmetxqa | 
| 4 | 4  | 666623  | lightblue | 
| 5 | 5  | 542123  | foobarlight | 

能有人告訴我，我應該如何進行查詢，檢查是否所有4貓的是匹配的，然後給的我cat'id？我試過這種方式，但我認爲這是錯誤的方式，因爲每次產品只有2或3只貓，我沒有得到相關的catId。所以它只適用於所有4只貓定義的產品。

SELECT 
    cat.`id`, 
    prod.`productnr`, 
    prod.`productname` 
FROM 
    products as prod 
LEFT JOIN 
    categorys as cat 
ON 
    cat.`cat1` = prod.`cat1` 
AND 
    cat.`cat2` = prod.`cat2` 
AND 
    cat.`cat3` = prod.`cat3` 
AND 
    cat.`cat4` = prod.`cat4` 

如果有人對我也有用tipps，請告訴我。 ;-)

感謝您幫助我:)

Answer 1

可以接近這個通過爲每個類別組合分別進行連接。當然，這是一個層次排名，你不想重複。因此，下面做這些檢查：

SELECT prod.id, coalesce(c4.id, c3.id, c2.id, c1.id) as catid 
     prod.`productnr`, prod.`productname` 
FROM products prod left join 
    categorys c4 
    on c4.cat1 = prod.cat1 and c4.cat2 = prod.cat2 and 
     c4.cat3 = prod.cat3 and c4.cat4 = prod.cat4 left join 
    categorys c3 
    on c3.cat1 = prod.cat1 and c3.cat2 = prod.cat2 and 
     c3.cat3 = prod.cat3 and c3.cat4 is null and 
     c4.id is null left join 
    categorys c2 
    on c2.cat1 = prod.cat1 and c2.cat2 = prod.cat2 and c2.cat3 is null and 
     c3.id is null and c4.id is null left join 
    categorys c1 
    on c1.cat1 = prod.cat1 and c1.cat2 is null and 
     c2.id is null and c3.id is null and c4.id is null; 

這是可能的，這將在某些情況下產生重複的行（雖然它的工作原理，以避免這種情況）。如果發生這種情況，那麼group by可能仍然是必要的。

Answer 2

首先，您需要正確的數據模型。

分類：

| ID | catname  | 
+----+--------------+ 

產品：

| ID | productnr | productname | 
+----+-------------+---------------+ 

而且是如此的神奇結表：

|ID | product_id | category_id| 
+---+------------+------------+ 

現在可以正確地問你的數據庫，爲您提供數據

SELECT 
    cat.`id`, 
    prod.`productnr`, 
    prod.`productname` 

FROM categories as cat 

INNER JOIN products2categories p2c 
ON p2c.category_id = cat.id 

INNER JOIN products prod 
ON p2c.product_id = prod.id 

因爲我真的不喜歡發放完整的複製粘貼示例，所以我會讓您使用上面的查詢來查看如何找到存在於4個類別中的產品。請記住 - 最簡單的方法通常總是正確的方法。

Answer 3

如果您只想在每個cat字段加入時獲得id s，請嘗試使用INNER JOIN而不是LEFT JOIN。

SELECT 
    cat.`id`, 
    prod.`productnr`, 
    prod.`productname` 
FROM 
    products as prod 
INNER JOIN 
    categorys as cat 
ON 
    cat.`cat1` = prod.`cat1` 
AND 
    cat.`cat2` = prod.`cat2` 
AND 
    cat.`cat3` = prod.`cat3` 
AND 
    cat.`cat4` = prod.`cat4` 

Answer 4

的問題很可能在你的桌子空，因爲空！= NULL在SQL

來繞去這個問題，你可以合併數據

SELECT 
    cat.`id`, 
    prod.`productnr`, 
    prod.`productname` 
FROM 
    products as prod 
LEFT JOIN 
    categorys as cat 
ON 
    coalesce(cat.`cat1`,'No Value') = coalesce(prod.`cat1`,'No Value') 
AND 
    coalesce(cat.`cat2`,'No Value') = coalesce(prod.`cat2`,'No Value') 
AND 
    coalesce(cat.`cat3`,'No Value') = coalesce(prod.`cat3`,'No Value') 
AND 
    coalesce(cat.`cat4`,'No Value') = coalesce(prod.`cat4`,'No Value')