Question

我在mysql中有三个表：

1.t_item
    itemID    item_name     
      1        pen      
      2        luxury pen       


2.s_item
      itemID    item_name     
      1          shoes
      2          clothes
      3          computer


3.track
      trackID    item_no   item_type  
      1           1         t_item
      2           2         t_item
      3           2         s_item

我想获得trackID，表轨道中item_no的项目名称。如何编写sql语句？在track中，trackID 2,3具有相同的item_no但不同的项类型，我可以使用join吗？

Answer 1

你走了，它会起作用：

(SELECT t.trackID,ti.item_name 
    FROM track t 
    LEFT JOIN t_item ti 
    ON t.item_no=ti.itemID AND t.item_type='t_item') 
UNION
(SELECT t.trackID,si.item_name 
    FROM track t 
    LEFT JOIN s_item si 
    ON t.item_no=si.itemID AND t.item_type='s_item')

Answer 2

是的，您可以像这样使用JOIN：

SELECT t.trackID, t.item_no, 
  (CASE WHEN t.item_type = 't_item' THEN ti.item_name
       WHEN t.item_type = 's_item' THEN si.item_name
  END) AS item_name
FROM track t
  LEFT JOIN t_item ti ON t.item_no = ti.itemID AND t.item_type = 't_item'
  LEFT JOIN s_item si ON t.item_no = si.itemID AND t.item_type = 's_item';

这是sqlfiddle。

Answer 3

你在这里：

(SELECT track.trackID, t_item.item_name FROM track
    LEFT JOIN t_item ON track.item_no=t_item.itemID 
    WHERE track.item_type = 't_item') 
UNION
(SELECT track.trackID, s_item.item_name FROM track
    LEFT JOIN s_item ON track.item_no=s_item.itemID 
    WHERE track.item_type = 's_item')

MySQL：在三个表中选择数据

3 个答案: