[英]SELECT rows as columns in MySQL?
考慮下表“視圖”
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
如您所見,該表包含每個用戶 (user_id) 每天 (_date) 的綜合瀏覽量 (cnt)。 我正在尋找一個 SELECT 查詢,它將 output user_ids 作為列,因此表數據將采用矩陣形式,如下所示:
_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
這可能與 SELECT 語句有關嗎?
如果你正在處理一組有限的用戶 ID,你可以這樣做:
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
不過,與其說這是一個好的查詢,不如說是一種技巧。
看起來您有一長串要轉換的值。 如果是這種情況,那么您可以使用准備好的語句。 您的代碼將如下所示(請參閱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;
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