“Simple” SQL Query

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 eac

Answer 1

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 2

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 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

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

Answer 5

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