Select one row from non unique rows based on row value - sql

Question

I have a quiz table
id | user_id | quiz_id
--------------------------
1 | 34567 | 12334
2 | 34567 | 12334
3 | 34567 | 23455
id 1 and 2 depicts a quiz that can be assigned to the same user twice
and a quiz transaction table
id | date | status
------------------------
1 | 2014 | assigned
2 | 2014 | assigned
3 | 2014 | assigned
------------------------
1 | 2014 | completed
id is foreign key to quiz table id, the last row depicts whenever a user finished the quiz, the row in the transaction table is updated with status 'completed'
Expected Result: I want a table with a structure like
id | user_id| course_id | date | status
------------------------------------------
1 | 34567 | 12334 | 2014 | completed
2 | 32567 | 12334 | 2014 | assigned
3 | 2014 | 23455 | 2014 | assigned
My query is
SELECT q.id, q.user_id, q.course_id, qt.date, qt.status FROM quiz q
LEFT JOIN
quiz_transaction qt ON
q.id = qt.id
but it gives me extra row (as the query will)
1 | 34567 | 12334 | 2014 | assigned
I cannot use
ON qt.type = 'completed'
Because if its completed it should return a completed row and if not it should return an assigned row but not both.
So in the result I cannot have
1 | 34567 | 12334 | 2014 | completed
1 | 34567 | 12334 | 2014 | assigned
How can I do it?

Answer 1

How about simply using the MAX() function with GROUP BY (SQL Fiddle):
SELECT q.id, q.user_id, q.course_id, qt.date, MAX(qt.status) AS Status
FROM quiz q
LEFT JOIN quiz_transaction qt ON q.id = qt.id
GROUP BY q.id, q.user_id, q.course_id, qt.date
EDIT: If you need to order a string a certain way, you could use a CASE statement to convert the string to a number. Get the MAX value and then convert it back (SQL Fiddle):
SELECT m.id, m.user_id, m.quiz_id, MAX(m.date),
CASE WHEN MAX(m.status) = 1 THEN 'assigned'
WHEN MAX(m.status) = 2 THEN 'doing'
WHEN MAX(m.status) = 3 THEN 'completed' END AS Status
FROM
(
SELECT q.id, q.user_id, q.quiz_id, qt.date,
CASE WHEN qt.status = 'assigned' THEN 1
WHEN qt.status = 'doing' THEN 2
WHEN qt.status = 'completed' THEN 3 END AS Status
FROM quiz q
LEFT JOIN quiz_transaction qt ON q.id = qt.id
) AS m
GROUP BY m.id, m.user_id, m.quiz_id;

Answer 2

Depending on your release SQLnServer supports Standard SQL's "Windowed Aggregate Functions". ROW_NUMBER will give you a single row:
SELECT
q.id
,q.user_id
,q.quiz_id
,qt.date
,qt.status
FROM quiz q
JOIN
(
SELECT
id
,date
,status
,ROW_NUMBER()
OVER (PARTITION BY id
ORDER BY Status DESC) as rn
FROM quiz_transaction
) as qt
ON q.id = qt.id
WHERE rn = 1
If you got more complex ordering rules you need to use a CASE:
,ROW_NUMBER()
OVER (PARTITION BY id
ORDER BY CASE Status WHEN 'completed' THEN 1
WHEN 'doing' THEN 2
WHEN 'assigned' THEN 3
END) as rn

Answer 3

try this
SELECT q.id, q.user_id, q.course_id, q1.date, qt.status FROM quiz q
LEFT JOIN
(Select id , convert(varchar,max(convert(varbinary,status ))) 'Status'
from quiz_transaction
group by id
) qt ON
q.id = qt.id
left join quiz_transaction q1 on q1.id = qt.id and q1.status=qt.status