Question

我有一个类似

的查询

WITH a AS ( SELECT * 
              FROM inventory_tagalongs 
             WHERE TagAlong_ItemNum <> 'bokiwi2'
          )

SELECT inventory.itemnum
     , inventory.itemname
     , inventory.ItemType,inventory.dept_id  
  FROM inventory 
  LEFT OUTER JOIN a 
    ON inventory.itemnum = a.itemnum
  JOIN departments
    ON inventory.dept_id = departments.dept_id
  JOIN categories 
    ON departments.subtype = categories.cat_id AND categories.description = 'vapors'

我试图从较低的a中排除syntax的结果，但是无论left outer join.....是否存在，返回的结果都完全相同。

我的语法不正确吗？

Answer 1

下午好。请尝试以下方法：

select 
   inventory.itemnum
   , inventory.itemname
   , inventory.ItemType
   , inventory.dept_id 
from inventory 
join departments on inventory.dept_id=departments.dept_id 
join categories on departments.subtype=categories.cat_id and categories.description='vapors'
WHERE NOT EXISTS(
  select 1 from inventory_tagalongs B where inventory.itemnum = b.itemnum 
  AND B.TagAlong_ItemNum <>'bokiwi2');

CTE对此问题有所思考。这样可以通过易于阅读的代码实现目标，并且性能也应更好。

谢谢约翰。

Answer 2

如果您试图获得没有标签的结果，则逻辑将被反转。您想要：

SELECT i.itemnum, i.itemname, i.ItemType, i.dept_id  
FROM inventory i JOIN
     departments d
     ON i.dept_id = d.dept_id JOIN
     categories c
     ON d.subtype = c.cat_id AND
        c.description = 'vapors' LEFT OUTER JOIN
     a 
     ON i.itemnum = a.itemnum
WHERE a.itemnum IS NULL;

您根本不需要CTE。通常写为：

WITH a AS (

    )
SELECT i.itemnum, i.itemname, i.ItemType, i.dept_id  
FROM inventory i JOIN
     departments d
     ON i.dept_id = d.dept_id JOIN
     categories c
     ON d.subtype = c.cat_id AND
        c.description = 'vapors' LEFT OUTER JOIN
     inventory_tagalongs it
     ON i.itemnum = it.itemnum AND it.TagAlong_ItemNum = 'bokiwi2' 
WHERE a.itemnum IS NULL;

而且-如另一个答案所述-NOT EXISTS是解决此问题的另一种典型方法。

左外联接返回结果被排除

2 个答案: