MySQL的加入兩個地方

Question

兩個表這是我的查詢：

SELECT a.id, a.hostname, a.asset_tag, d.model, b.location, 
     c.type, a.earmarked, a.earmarked_office, a.earmarked_user, 
     a.earmarked_date, a.earmarked_by 
     FROM laptops a 
     JOIN locations b ON a.location = b.id 
     JOIN types c ON a.type = c.id 
     JOIN models d ON a.model = d.id 
     WHERE b.stock = 1 

現在，a.earmarked_office場實際上是在locations表中的ID，這是我們已經在使用，填補了b.location參考。我怎樣才能在相同的聲明中檢索earmarked_office的實際位置名稱？

Answer 1

加入表兩次：

SELECT lt.id, lt.hostname, lt.asset_tag, d.model, l.location, 
     c.type, lt.earmarked, lt.earmarked_office, lt.earmarked_user, 
     lt.earmarked_date, lt.earmarked_by, 
     lteo.?? 
FROM laptops lt JOIN 
    locations l 
    ON lt.location = l.id JOIN 
    types t 
    ON lt.type = t.id JOIN 
    models m 
    ON lt.model = m.id JOIN 
    locations leo 
    ON lt.earmarked_office = leo.id 
WHERE l.stock = 1; 

注：

• 使用表的別名是好的。但是，使用表別名可以更輕鬆地遵循查詢的邏輯。
• 我不知道你想從第二張表中得到什麼列。
• 使用as重命名列，因此它們不會列已經從l未來衝突。
• 如果存在不匹配的值，則可能需要LEFT JOIN。

Answer 2

您可以加入同一個表多次：

SELECT a.id, a.hostname, a.asset_tag, d.model, b.location, 
     c.type, a.earmarked, a.earmarked_office, a.earmarked_user, 
     a.earmarked_date, a.earmarked_by 
     FROM laptops a 
     JOIN locations b ON a.location = b.id 
     JOIN types c ON a.type = c.id 
     JOIN models d ON a.model = d.id 
     JOIN locations e ON a.earmarked_office = e.id 
     WHERE b.stock = 1