使用循环连接语法到左表并列出列[关闭]

Question

我有那 3 张桌子

Warehouse

Id  name        
1   Warehouse 1     
2   Warehouse 2     


Items

Id  description     
1   Item 1      
2   Item 2      
3   Item 3



itemmovement

itemid     qtyin    qtyout  warehouseid
1            2                   1
1                      1         1
1            2                   2
2            1                   1
2            3                   1
2                      1         1
2            1                   2


Result      
ItemId  SumQuantityWarehouse1   SumQuantityWarehouse2
1              1                         2
2              3                         1
3              0                         0

我需要通过在列中列出所有仓库以及数量 结果中显示的每个项目

Answer 1

在 MySQL 中实现这一点的唯一实用方法是使用存储过程来构建基于仓库列表的动态查询。查询必须使用条件聚合来构建结果表。对于您的示例数据，构建的查询如下所示：

SELECT i.Id
     , SUM(CASE WHEN m.warehouseID=1 THEN COALESCE(m.qtyin, 0) - COALESCE(m.qtyout, 0) ELSE 0 END) AS `SUMWarehouse 1`
     , SUM(CASE WHEN m.warehouseID=2 THEN COALESCE(m.qtyin, 0) - COALESCE(m.qtyout, 0) ELSE 0 END) AS `SUMWarehouse 2`
FROM items i 
LEFT JOIN itemmovement m ON m.itemid = i.Id
GROUP BY Id

程序如下：

DELIMITER //
DROP PROCEDURE IF EXISTS stocktake //
CREATE PROCEDURE stocktake()
BEGIN
  DECLARE wid INT;
  DECLARE wname VARCHAR(20);
  DECLARE query TEXT DEFAULT '';
  DECLARE finished INT DEFAULT 0;
  DECLARE whouse_cursor CURSOR FOR SELECT Id, name FROM warehouse;
  DECLARE CONTINUE HANDLER FOR NOT FOUND SET finished = 1;
  OPEN whouse_cursor;
  SET @query = 'SELECT i.Id';
get_whouse: LOOP
  FETCH whouse_cursor INTO wid, wname;
  IF finished = 1 THEN 
    LEAVE get_whouse;
  END IF;
  SET @query = CONCAT(@query, ', SUM(CASE WHEN m.warehouseID=', wid, ' THEN COALESCE(m.qtyin, 0) - COALESCE(m.qtyout, 0) ELSE 0 END) AS `', wname, '`');
  END LOOP get_whouse;
  SET @query = CONCAT(@query, ' FROM items i LEFT JOIN itemmovement m ON m.itemid = i.Id');
  SET @query = CONCAT(@query, ' GROUP BY Id');
  PREPARE stmt FROM @query;
  EXECUTE stmt;
  DEALLOCATE PREPARE stmt;
END //
DELIMITER ;

以及您的示例数据的输出：

Id  Warehouse 1     Warehouse 2
1   1               2
2   3               1
3   0               0

Demo on dbfiddle

Alternate demo 展示了使用 5 个仓库的过程。