SQL每个ID仅选择一次，按最早的日期时间选择

Question

I'm currently having to run a query like the below for a one off report process at work.

However for each item in the table there are multiple associated "messages" that are all saved, this means each item is returned multiple times. I'd like to only show each item once, as per the examples and further explanation below.

I realize this is (at least in my opinion) a poor structure, but the report needs to be done and this is how the data is stored :-(

SQL Fiddle: http://sqlfiddle.com/#!6/76fce/8

Query:

SELECT messageId, receiver, createdDate, itemId from messages_0,items WHERE
    itemId IN (1, 2, 3)
    AND (receiver = '100' OR receiver = '200')
    AND messages_0.description LIKE '%'+items.name+'%'
union all
SELECT messageId, receiver, createdDate, itemId  from messages_1,items WHERE
    itemId IN (1, 2, 3)
    AND (receiver = '100' OR receiver = '200')
    AND messages_1.description LIKE '%'+items.name+'%'

Note: there are two message tables, hence the union all

Example messages:

messageId | receiver | createdDate       | description
--------------
1         | 100      | 2012/11/27 12:00  | The Dog is awesome
2         | 100      | 2012/11/27 13:00  | Now the Dog is boring
4         | 200      | 2012/11/27 11:30  | I have Wood :-)

Example items:

itemID | name 
--------------
1      | Dave
2      | Dog
3      | Wood

Result:

messageId | receiver | createdDate      | itemId 
1         | 100      | 2012/11/27 12:00 | 2
2         | 100      | 2012/11/27 13:00 | 2
4         | 200      | 2012/11/27 11:00 | 3

However, I need to only show each item once. Where only the oldest row (by the createdDate) is shown.

Target Result:

messageId | receiver | createdDate      | itemId 
1         | 100      | 2012/11/27 12:00 | 2
4         | 200      | 2012/11/27 11:00 | 3

How can I do this in SQL (Sybase)? So far I have been looking at both group by (which would only return an id) and some sort of sub query, but have been unable to get anything to work!

SQL Fiddle: http://sqlfiddle.com/#!6/76fce/8

Answer 1

If I understood you right, something like this could be a start.

SELECT
  t, messageId, receiver, createdDate, itemId 
FROM
  (
    SELECT 
      m.messageId, m.receiver, m.createdDate, m.t,
      i.itemId 
    FROM
      items i
      INNER JOIN (
        SELECT description, messageId, receiver, createdDate, 0 t FROM messages_0 
        UNION 
        SELECT description, messageId, receiver, createdDate, 1 t FROM messages_1
      ) m ON m.description LIKE '%' + i.name + '%' 
             AND m.receiver IN ('100', '200')
    WHERE
      i.itemId IN (1, 2, 3)
  ) data
WHERE
  createdDate = (
    SELECT MIN(createdDate) FROM (
      SELECT createdDate FROM messages_0 WHERE messageId = data.messageId AND data.t = 0
      UNION
      SELECT createdDate FROM messages_1 WHERE messageId = data.messageId AND data.t = 1
    )
  )

I would put indexes on

• messages_0 / messages_1
  • (messageId, createdDate)
  • (receiver, messageId, createdDate, description)
• items
  • (itemId, name)