Question

我想创建组查询，其中Table值如下所示：

EMP_ID  ProjectID
815     1
985     1
815     3
985     4
815     4

我想要这样的输出

EMP_ID ProjectID1 ProjectID2 ProjectID3
815    1          3          4
985    1          4          0

任何人都知道如何在SQL查询中实现这一点。

提前感谢。

Answer 1

简短的方法：

使用http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_group-concat

SELECT
  tbl.emp_id,
  GROUP_CONCAT( DISTINCT project_id ) project_id_list
FROM tbl
GROUP BY tbl.emp_id

在这种情况下，您必须在应用程序中拆分/处理连接的project_id_list字符串（或NULL）

很长的路要走：

我们将使用一个小技巧：

http://dev.mysql.com/doc/refman/5.1/en/example-auto-increment.html

对于MyISAM表，您可以在辅助列上指定AUTO_INCREMENT 在多列索引中。在这种情况下，生成的值为 AUTO_INCREMENT列计算为MAX（auto_increment_column）+ 1 WHERE prefix = given-prefix。当您想要放置数据时，这非常有用进入有序的群体。

CREATE TEMPORARY TABLE temp (
  emp_id      INT NOT NULL,
  -- project_num will count from 1 to N PER emp_id!
  project_num INT NOT NULL AUTO_INCREMENT,
  project_id  INT NOT NULL,
  PRIMARY KEY ( emp_id, project_num )
) ENGINE=MyISAM; -- works only with myisam!

生成每组自动增量：

INSERT INTO temp ( emp_id, project_id )
SELECT emp_id, project_id FROM tbl

计算需要多少project_id列：

$MAX_PROJECTS_PER_EMP =
  SELECT MAX( max_projects_per_emp ) FROM
    ( SELECT COUNT(*) AS max_projects_per_emp project_id FROM tbl GROUP BY emp_id )

以编程方式创建选择表达式：

SELECT
  temp.emp_id,
  t1.project_id  AS project_id_1,
  t2.project_id  AS project_id_2,
  t98.project_id AS project_id_98,
  t99.project_id AS project_id_99,
FROM      temp
LEFT JOIN temp AS t1 ON temp.emp_id = t1.id AND t1.project_num = 1
LEFT JOIN temp AS t2 ON temp.emp_id = t2.id AND t1.project_num = 2
// create $MAX_PROJECTS_PER_EMP lines of LEFT JOINs
LEFT JOIN temp AS t98 ON temp.emp_id = t98.id AND t98.project_num = 98
LEFT JOIN temp AS t99 ON temp.emp_id = t99.id AND t99.project_num = 99

SQL Group By和Display with Different Values

1 个答案:

简短的方法：

很长的路要走：