How does DISTINCT interact with ORDER BY?

问题

Consider the two tables below:

user:
ID | name
---+--------
1  | Alice
2  | Bob
3  | Charlie

event:
order | user
------+------------
1     | 1 (Alice)
2     | 2 (Bob)
3     | 3 (Charlie)
4     | 3 (Charlie)
5     | 2 (Bob)
6     | 1 (Alice)

If I run the following query:

SELECT DISTINCT user FROM event ORDER BY "order" DESC;

will it be guaranteed that I get the results in the following order?

1 (Alice)
2 (Bob)
3 (Charlie)

If the three last rows of event are selected, I know this is the order I get, because it would be ordering 4, 5, 6 in descending order. But if the first three rows are selected, and then DISTINCT prevents the last tree to be loaded for consideration, I would get it in reversed order.

Is this behavior well defined in SQL? Which of the two will happen? What about in SQLite?

回答1:

No, it will not be guaranteed.
Find Itzik Ben-Gan's Logical Query Processing Phases poster for MS SQL. It migrates over many sites, currently found at https://accessexperts.com/wp-content/uploads/2015/07/Logical-Query-Processing-Poster.pdf . DISTINCT preceeds ORDER BY .. TOP and Sql Server is free to return any of 1 | 1 (Alice) or 6 | 1 (Alice) rows for Alice. So any of (1,2,3), (1,4,5) an so on are valid results of DISTINCT.

回答2:

Here's a query solution that I believe solves your problem.

SELECT 
    MAX([order]) AS MaxOrd
  , [user]
FROM Event
GROUP BY [User]
ORDER BY MaxOrd DESC

来源：https://stackoverflow.com/questions/37257746/how-does-distinct-interact-with-order-by