Question

I have views which look something like this:

mysql> select * from p2;
+---+---------+---------+
| k | measure | time_id |
+---+---------+---------+
| D |     200 |       2 |
| E |     201 |       2 |
| F |     203 |       2 |
| A |      20 |       1 |
| B |      22 |       1 |
| C |      23 |       1 |
| D |     100 |       1 |
| E |     101 |       1 |
| F |     103 |       1 |
| G |       4 |       1 |
| H |       7 |       1 |
| I |      10 |       1 |
+---+---------+---------+

(k, time_id) is a unique key, and the above is greatly simplified (there will be many more values of time_id and k). The sort order is time_id DESC (followed by k ASC, but that’s not so important).

I want to find a SELECT statement that will filter it to this:

+---+---------+---------+
| k | measure | time_id |
+---+---------+---------+
| D |     200 |       2 |
| E |     201 |       2 |
| F |     203 |       2 |
| A |      20 |       1 |
| B |      22 |       1 |
| C |      23 |       1 |
| G |       4 |       1 |
| H |       7 |       1 |
| I |      10 |       1 |
+---+---------+---------+

I want to make sure that values for column k are unique, by filtering out rows where the k value has already been used before.

In this example, in the original view rows 0, 1, 2 contained k values D, E and F, but so did rows 6, 7, 8, so rows 6-8 are removed to make the second view.

Is there a SELECT statement that can do this? It feels like it should be straightforward, but I can’t figure out how to do it.

Answer 1

You may want to use a derived table:

SELECT p2.* 
FROM   p2
JOIN   (
           SELECT    MAX(time_id) max_time, k 
           FROM      p2 
           GROUP BY  k
       ) d_p2 ON (d_p2.k = p2.k AND d_p2.max_time = p2.time_id);

Or you could also use the “null-self-join” method:

SELECT    p2.*
FROM      p2
LEFT JOIN p2 AS d_p2 ON d_p2.k = p2.k AND d_p2.time_id > p2.time_id
WHERE     d_p2.k IS NULL;

These should work fine as long as you are sure that time_id is unique for each k. Otherwise you could still get duplicate rows.

Test case:

CREATE TABLE p2 (k char(1), measure int, time_id int);

INSERT INTO p2 VALUES ('D', 200, 2);
INSERT INTO p2 VALUES ('E', 201, 2);
INSERT INTO p2 VALUES ('F', 203, 2);
INSERT INTO p2 VALUES ('A',  20, 1);
INSERT INTO p2 VALUES ('B',  22, 1);
INSERT INTO p2 VALUES ('C',  23, 1);
INSERT INTO p2 VALUES ('D', 100, 1);
INSERT INTO p2 VALUES ('E', 101, 1);
INSERT INTO p2 VALUES ('F', 103, 1);
INSERT INTO p2 VALUES ('G',   4, 1);
INSERT INTO p2 VALUES ('H',   7, 1);
INSERT INTO p2 VALUES ('I',  10, 1);

Result:

+------+---------+---------+
| k    | measure | time_id |
+------+---------+---------+
| D    |     200 |       2 |
| E    |     201 |       2 |
| F    |     203 |       2 |
| A    |      20 |       1 |
| B    |      22 |       1 |
| C    |      23 |       1 |
| G    |       4 |       1 |
| H    |       7 |       1 |
| I    |      10 |       1 |
+------+---------+---------+
9 rows in set (0.00 sec)