SQL to create matrix table

问题:

I want to make a view in MySQL that will be generated from 3 tables.

Please see the following tables.

All I want to do is to create a view, using col:name of table:ItemList as the labels of the columns of the new view.

How can I achieve this using SQL?

Thank you a lot in advance.

table: ItemList ,this changes so frequently

+----+-------------+
| id | name        |
+----+-------------+
|  1 | Apple       |
|  2 | Orange      |
|  3 | Banana      |
|  4 | Kiwi        |
|  5 | Mango       |
+----+-------------+

table: UserList

+----+-------------+
| id | name        |
+----+-------------+
|  1 | John        |
|  2 | Mary        |
|  3 | James       |
+----+-------------+

table: OrderList

+----+------+------+-----+
| id | User | Item | qty |
+----+------+------+-----+
|  1 |    1 |    4 |   1 |
|  2 |    1 |    2 |   2 |
|  3 |    2 |    1 |   4 |
|  4 |    1 |    3 |   3 |
|  5 |    3 |    5 |   1 |
|  6 |    2 |    2 |   2 |
+----+------+------+-----+

view that I want to create

+-------+-------+--------+--------+------+-------+
| User  | Apple | Orange | Banana | Kiwi | Mango |
+-------+-------+--------+--------+------+-------+
| John  |       |      2 |      3 |    1 |       |
| Mary  |     4 |      2 |        |      |       |
| James |       |        |        |      |     1 |
+-------+-------+--------+--------+------+-------+

回答1:

You have to use conditional summation and dynamic SQL for this

SET @sql = NULL;
SELECT
  GROUP_CONCAT(DISTINCT
    CONCAT(
      'SUM(CASE WHEN l.Item = ',
      id,
      ' THEN l.qty END) `',
      name, '`'
    )
  ) INTO @sql
FROM ItemLIst;

SET @sql = CONCAT('SELECT u.name, ', @sql, ' 
                     FROM OrderList l JOIN UserList u
                       ON l.User = u.id
                    GROUP BY u.name');

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

Output:

|  NAME |  APPLE | ORANGE | BANANA |   KIWI |  MANGO |
------------------------------------------------------
| James | (null) | (null) | (null) | (null) |      1 |
|  John | (null) |      2 |      3 |      1 | (null) |
|  Mary |      4 |      2 | (null) | (null) | (null) |

Here is SQLFiddle demo

Now you won't be able to wrap it into a view, but you can make it a stored procedure.

DELIMITER $$
CREATE PROCEDURE sp_order_report()
BEGIN
  SET @sql = NULL;
  SELECT
    GROUP_CONCAT(DISTINCT
      CONCAT(
        'SUM(CASE WHEN l.Item = ',
        id,
        ' THEN l.qty END) `',
        name, '`'
      )
    ) INTO @sql
  FROM
    ItemLIst;
  SET @sql = CONCAT('SELECT u.name, ', @sql, ' 
                       FROM OrderList l JOIN UserList u
                         ON l.User = u.id
                      GROUP BY u.name');

  PREPARE stmt FROM @sql;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
END$$
DELIMITER ;

And use it like this:

CALL sp_order_report();

Here is SQLFiddle demo