"Simple" SQL Query - sql

Question

Each of my clients can have many todo items and every todo item has a due date.
What would be the query for discovering the next undone todo item by due date for each file? In the event that a client has more than one todo, the one with the lowest id is the correct one.
Assuming the following minimal schema:
clients (id, name)
todos (id, client_id, description, timestamp_due, timestamp_completed)
Thank you.

Answer 1

I haven't tested this yet, so you may have to tweak it:
SELECT
TD1.client_id,
TD1.id,
TD1.description,
TD1.timestamp_due
FROM
Todos TD1
LEFT OUTER JOIN Todos TD2 ON
TD2.client_id = TD1.client_id AND
TD2.timestamp_completed IS NULL AND
(
TD2.timestamp_due < TD1.timestamp_due OR
(TD2.timestamp_due = TD1.timestamp_due AND TD2.id < TD1.id)
)
WHERE
TD2.id IS NULL
Instead of trying to sort and aggregate, you're basically answering the question, "Is there any other todo that would come before this one?" (based on your definition of "before"). If not, then this is the one that you want.
This should be valid on most SQL platforms.

Answer 2

This question is the classic pick-a-winner for each group. It gets posted about twice a day.
SELECT *
FROM todos t
WHERE t.timestamp_completed is null
and
(
SELECT top 1 t2.id
FROM todos t2
WHERE t.client_id = t2.client_id
and t2.timestamp_completed is null
--there is no earlier record
and
(t.timestamp_due > t2.timestamp_due
or (t.timestamp_due = t2.timestamp_due and t.id > t2.id)
)
) is null

Answer 3

SELECT c.name, MIN(t.id)
FROM clients c, todos t
WHERE c.id = t.client_id AND t.timestamp_complete IS NULL
GROUP BY c.id
HAVING t.timestamp_due <= MIN(t.timestamp_due)
Avoids a subquery, correlated or otherwise but introduces a bunch of aggregate operations which aren't much better.

Answer 4

Some Jet SQL, I realize it is unlikely that the questioner is using Jet, however the reader may be.
SELECT c.name, t.description, t.timestamp_due
FROM (clients c
INNER JOIN
(SELECT t.client_id, Min(t.id) AS MinOfid
FROM todos t
WHERE t.timestamp_completed Is Null
GROUP BY t.client_id) AS tm
ON c.id = tm.client_id)
INNER JOIN todos t ON tm.MinOfid = t.id

Answer 5

The following should get you close, first get the min time for each client, then lookup the client/todo information
SELECT
C.Id,
C.Name,
T.Id
T.Description,
T.timestamp_due
FROM
{
SELECT
client_id,
MIN(timestamp_due) AS "DueDate"
FROM todos
WHERE timestamp_completed IS NULL
GROUP BY ClientId
} AS MinValues
INNER JOIN Clients C
ON (MinValues.client_id = C.Id)
INNER JOIN todos T
ON (MinValues.client_id = T.client_id
AND MinValues.DueDate = T.timestamp_due)
ORDER BY C.Name
NOTE: Written assuming SQL Server