SELECT 行作为 MySQL 中的列？

Question

Consider the following table "views"

user_id  _date       cnt
------------------------
1        2011-02-10  123
1        2011-02-11   99
1        2011-02-12  100
1        2011-02 13   12
2        2011-02-10   32
2        2011-02-11  433
2        2011-02-12  222
2        2011-02 13  334
3        2011-02-10  766
3        2011-02-11  654
3        2011-02-12   43
3        2011-02 13   27
...
100      2011-02-13  235

As you can see, the table holds pageviews (cnt) per day (_date) per user (user_id). I'm looking for a SELECT query that will output the user_ids as columns so the table-data will be in matrix form, as follows:

_date         1    2    3 ... 100
---------------------------------
2011-02-10  123   32  766
2011-02-11   99  433  654
2011-02-12  100  222   43
2011-02-13   12  334   27     235

Is this possible to do with a SELECT statement?

Answer 1

If you're dealing with a finite set of user IDs, you could do something like this:

SELECT _date,
    SUM(CASE WHEN _user_id = 1 THEN cnt ELSE 0 END) AS user1,
    SUM(CASE WHEN _user_id = 2 THEN cnt ELSE 0 END) AS user2,
    SUM(CASE WHEN _user_id = 3 THEN cnt ELSE 0 END) AS user3,
    ...
FROM views
GROUP BY _date

It's more of a hack than a good query, though.

Answer 2

It looks like you have an long list of values that you want to transform. If that is the case then you can use prepared statements . Your code will look like this (see SQL Fiddle with Demo ):

CREATE TABLE Table1
    (`user_id` int, `_date` datetime, `cnt` int)
;

INSERT INTO Table1
    (`user_id`, `_date`, `cnt`)
VALUES
    (1, '2011-02-09 17:00:00', 123),
    (1, '2011-02-10 17:00:00', 99),
    (1, '2011-02-11 17:00:00', 100),
    (1, '2011-02-13 00:00:00', 12),
    (2, '2011-02-09 17:00:00', 32),
    (2, '2011-02-10 17:00:00', 433),
    (2, '2011-02-11 17:00:00', 222),
    (2, '2011-02-13 00:00:00', 334),
    (3, '2011-02-09 17:00:00', 766),
    (3, '2011-02-10 17:00:00', 654),
    (3, '2011-02-11 17:00:00', 43),
    (3, '2011-02-13 00:00:00', 27),
    (100, '2011-02-12 17:00:00', 235)
;

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'sum(case when user_id = ''',
      user_id,
      '''  then cnt else 0 end) AS ''',
      user_id, ''''
    )
  ) INTO @sql
FROM Table1;

SET @sql = CONCAT('SELECT _Date, ', @sql, ' 
                  FROM table1 
                  GROUP BY _Date');

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;