使用基本的sql查詢幫助

Question

該示例已簡化。我有以下設計： http://img835.imageshack.us/i/designyi.jpg/

我已經插入這樣的測試數據：

INSERT INTO Period VALUES ('Survey for 2011', 1) 

INSERT INTO EvalQuestion VALUES('How do...') 
INSERT INTO EvalQuestion VALUES('How many...') 
INSERT INTO EvalQuestion VALUES('Which is....') 

INSERT INTO EvalQuestion_Period VALUES (1, 1) 
INSERT INTO EvalQuestion_Period VALUES (1, 2) 
INSERT INTO EvalQuestion_Period VALUES (1, 3) 

INSERT INTO Employee VALUES ('Peter', 'Smith') 
INSERT INTO Employee VALUES ('Britney', 'Spears') 

INSERT INTO EvalAnswer VALUES(1,'Fine',1) 
INSERT INTO EvalAnswer VALUES(2,'45',1) 
INSERT INTO EvalAnswer VALUES(3,'I don´t know',1) 
INSERT INTO EvalAnswer VALUES(1,'Fine again',2) 
INSERT INTO EvalAnswer VALUES(2,'45 again',2) 
INSERT INTO EvalAnswer VALUES(3,'I don´t know again',2) 

我運行下面的查詢來獲取問題和答案彼得：

Select Name, Answer 
from EvalQuestion eq 
LEFT JOIN EvalQuestion_Period eqp ON eq.Id = eqp.FK_EvalQuestion 
LEFT JOIN EvalAnswer ea ON ea.FK_EvalQuestion_Period = eqp.Id 
where ea.FK_Employee = 1 

結果集：

Name   Answer 
----------------------- 
How do...  Fine 
How many... 45 
Which is.... I don´t know 

這看起來不錯。如果我刪除彼得斯的一個答案是這樣的：

Delete from EvalAnswer where ID= 1 

，並運行相同的查詢，我只得到兩行，像這樣

Name   Answer 
----------------------- 
How many... 45 
Which is.... I don´t know 

我需要我的問題的結果，即使設置爲只要是懸而未決，像這樣：

Name   Answer 
----------------------- 
How do....  NULL 
How many... 45 
Which is.... I don´t know 

任何提示？謝謝

Answer 1

你的「左連接」實際上是一個LEFT OUTER JOIN（儘管其他答案）：連接類型由LEFT或RIGHT與OUTER是可選

當您使用WHERE ea.FK_Employee = 1那麼你改變這個到內暗含加入，因爲你不允許缺少行。您需要首先進行過濾（即在加入前限制EvalAnswer上的行）。這是因爲WHERE在JOIN..ON邏輯後處理。

與派生，過濾表試試這個：

Select Name, Answer 
from EvalQuestion eq 
LEFT JOIN 
EvalQuestion_Period eqp ON eq.Id = eqp.FK_EvalQuestion 
LEFT JOIN 
     (SELECT * FROM EvalAnswer 
     where FK_Employee = 1 
     ) ea ON ea.FK_EvalQuestion_Period = eqp.Id 

或是在條件進行過濾：

Select Name, Answer 
from EvalQuestion eq 
    LEFT JOIN 
    EvalQuestion_Period eqp ON eq.Id = eqp.FK_EvalQuestion 
    LEFT JOIN 
    EvalAnswer ea ON ea.FK_EvalQuestion_Period = eqp.Id AND ea.FK_Employee = 1 

Answer 2

任何時候你想從原始表中返回一行或多行，你可以使用外連接。所以，你的查詢是：

Select Name, Answer 
    from EvalQuestion eq 
     LEFT JOIN EvalQuestion_Period eqp ON eq.Id = eqp.FK_EvalQuestion 
     LEFT OUTER JOIN EvalAnswer ea ON ea.FK_EvalQuestion_Period = eqp.Id 
    where ea.FK_Employee = 1 

這將返回空值從EvalAnswer值時，沒有相應的記錄存在，否則會正好充當LEFT JOIN。