在SELECT語句

Question

我做下面的SQL呼叫使用一個表多次：

SELECT dpt_id, 
dpt_title, 
dpt_active, 
dpt_parent 
FROM tbl_department ORDER BY dpt_title 

其中dpt_parent是上級部門ID（另一個部門的dpt_id）的整數值。

我希望上述調用的結果集包含該dpt_parent的dpt_title。我在這個初步嘗試如下：

SELECT dpt_id, 
dpt_title, 
dpt_active, 
dpt_parent, 
    (SELECT dpt_title 
    FROM tbl_department 
    WHERE tbl_department.dpt_id = dpt_parent 
    ) AS parent_title 
FROM tbl_department ORDER BY dpt_title 

有了這種形式的調用，PARENT_TITLE的所有值都爲空 我可以看到使用這樣的同桌是混亂的，該如何解決這個問題？

Answer 1

使用一個表的別名：

SELECT dpt_id, 
x.dpt_title, 
x.dpt_active, 
x.dpt_parent, 
    (SELECT dpt_title 
    FROM tbl_department 
    WHERE tbl_department.dpt_id = x.dpt_parent 
    ) AS parent_title 
FROM tbl_department x ORDER BY dpt_title 

，但你會更好刪除子選擇和剛剛加盟，如：

SELECT 
    a.dpt_id 
    ,a.dpt_title 
    ,a.dpt_active 
    ,a.dpt_parent 
    ,b.dpt_title parent_title 
FROM tbl_department  a 
LEFT JOIN tbl_department b ON a.dpt_parent = b.dpt_id 
ORDER BY a.dpt_title, b.dpt_title 

Answer 2

SELECT d.dpt_id, 
d.dpt_title, 
d.dpt_active, 
d.dpt_parent, 
p.dpt_title AS parent_title 
FROM tbl_department d 
LEFT JOIN tbl_department p 
    ON p.dpt_parent = d.dpt_id 
ORDER BY d.dpt_title 

Answer 3

SELECT d.dpt_id, 
d.dpt_title, 
d.dpt_active, 
d.dpt_parent, 
p.dpt_title parent_title 
FROM tbl_department d 
LEFT JOIN tbl_department p ON d.dpt_parent = p.dpt_id 
ORDER BY d.dpt_title 

Answer 4

我認爲解決的辦法是使參加同一個表或者你可以做一個用戶定義的函數，它的部門ID，並返回上級部門的名稱。

Answer 5

在我看來這個查詢需要被改寫爲：

SELECT dpt_id, 
dpt_title, 
dpt_active, 
dpt_parent, 
    (SELECT dpt_title 
    FROM tbl_department AS [dept2] 
    WHERE [dept2].dpt_id = [dept1].dpt_parent 
    ) AS parent_title 
FROM tbl_department AS [dept1] ORDER BY dpt_title 

否則SQL-server將不知道你正在談論的表的實例

Answer 6

我認爲你正在尋找類似的東西：

SELECT a.dpt_id, 
a.dpt_title, 
a.dpt_active, 
a.dpt_parent, 
b.dpt_title AS parent_title 
FROM tbl_department AS a 
INNER JOIN tbl_department AS b ON b.dpt_id = a.dpt_parent 
ORDER BY dpt_title