Help with basic sql query - sql

Question

This example is simplified. I have the following design:
http://img835.imageshack.us/i/designyi.jpg/
I have inserted test data like this:
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)
I run the following query to get question and answer for Peter:
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
Result set:
Name Answer
-----------------------
How do... Fine
How many... 45
Which is.... I don´t know
This looks good. If I delete one of Peters Answers like this:
Delete from EvalAnswer where ID= 1
And run the same query I only get two rows, like this
Name Answer
-----------------------
How many... 45
Which is.... I don´t know
I need my question in the result set even if it is unanswered, like this:
Name Answer
-----------------------
How do.... NULL
How many... 45
Which is.... I don´t know
Any tips? Thanks

Answer 1

Your "left join" is actually an LEFT OUTER JOIN (despite other answers): the join type is implied by LEFT or RIGHT with OUTER being optional
When you use WHERE ea.FK_Employee = 1 then you are changing this to an INNER JOIN because you are not allowing for missing rows. You need to filter first (that is restrict rows on EvalAnswer before the join). This is because WHERE is processed after JOIN..ON logically.
Try this with a derived, filtered table:
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
Or filter in the ON condition:
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

Any time you want to return 1 or more rows from the original table, you can use an outer join. So your query would be:
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
This will return nulls for the values from EvalAnswer when no corresponding records exist, but otherwise will function exactly as the LEFT JOIN.