一棵樹的SQL查詢的孫女

Question

誰能幫我寫出此特定輸出的sql代碼，請：

列出位於同一位置的樹（即tr_loc值相同），其中一棵樹是另一棵樹的孫女。 表數據

這是我想出的，但是我確定這是錯誤的：

SELECT tr_loc a, tr_loc b,  Tr_numb, Tr_forest 
FROM Tree
WHERE a.tr_loc = b.tr_loc 
  AND a.TR_parent = b.Tr_numb;

Answer 1

我認為這就是您要查詢的內容：

SELECT
a.tr_numb AS granddaughter
,c.tr_numb AS grandfather
,a.tr_loc
,a.Tr_forest

FROM
Tree a, Tree b, Tree c

WHERE
a.tr_parent = b.tr_numb
AND b.tr_parent = c.tr_numb
AND a.tr_forest = c.tr_forest
AND a.tr_loc = c.tr_loc

注意a ， b和c是表Tree別名。 那是一種自我加入 。 我將在此處保留該查詢，以便您可以嘗試查看哪里出錯了。

但是，我不建議使用該語法進行JOIN。 最好學習更明確的語法。 這是我編寫查詢的方式：

SELECT
Tree.Tr_numb AS tree
,Parent.Tr_numb AS parent
,Grandparent.Tr_numb AS grandparent
,Tree.tr_forest
,Tree.tr_loc

FROM
Tree
INNER JOIN Tree [Parent] ON (Parent.Tr_numb = Tree.Tr_parent)
INNER JOIN Tree [Grandparent] ON (Grandparent.Tr_numb = Parent.Tr_parent)

WHERE
Tree.Tr_loc = Grandparent.Tr_loc