替换从左外部联接返回的默认空值

Question

我有一个Microsoft SQL Server 2008查询,它使用左外连接从三个表返回数据.很多时候,第二个和第三个表中没有数据,所以我得到一个null,我认为这是左外连接的默认值.有没有办法替换select语句中的默认值？我有一个解决方法,我可以选择一个表变量,但感觉有点脏.

SELECT iar.Description, iai.Quantity, iai.Quantity * rpl.RegularPrice as 'Retail', 
iar.Compliance FROM InventoryAdjustmentReason iar
LEFT OUTER JOIN InventoryAdjustmentItem iai  on (iar.Id = iai.InventoryAdjustmentReasonId)
LEFT OUTER JOIN Item i on (i.Id = iai.ItemId)
LEFT OUTER JOIN ReportPriceLookup rpl on (rpl.SkuNumber = i.SkuNo)
WHERE iar.StoreUse = 'yes'

如果可能的话,我希望Quantity和RegularPrice默认为零.

Answer 1

这很简单

IsNull(FieldName, 0)

或者更完整:

SELECT iar.Description, 
  ISNULL(iai.Quantity,0) as Quantity, 
  ISNULL(iai.Quantity * rpl.RegularPrice,0) as 'Retail', 
  iar.Compliance 
FROM InventoryAdjustmentReason iar
LEFT OUTER JOIN InventoryAdjustmentItem iai  on (iar.Id = iai.InventoryAdjustmentReasonId)
LEFT OUTER JOIN Item i on (i.Id = iai.ItemId)
LEFT OUTER JOIN ReportPriceLookup rpl on (rpl.SkuNumber = i.SkuNo)
WHERE iar.StoreUse = 'yes'

Answer 2

如果是MySQL或SQLite，正确的关键字是IFNULL（not ISNULL）。

 SELECT iar.Description, 
      IFNULL(iai.Quantity,0) as Quantity, 
      IFNULL(iai.Quantity * rpl.RegularPrice,0) as 'Retail', 
      iar.Compliance 
    FROM InventoryAdjustmentReason iar
    LEFT OUTER JOIN InventoryAdjustmentItem iai  on (iar.Id = iai.InventoryAdjustmentReasonId)
    LEFT OUTER JOIN Item i on (i.Id = iai.ItemId)
    LEFT OUTER JOIN ReportPriceLookup rpl on (rpl.SkuNumber = i.SkuNo)
WHERE iar.StoreUse = 'yes'

Answer 3

的MySQL

COALESCE(field, 'default')

例如：

  SELECT
    t.id,
    COALESCE(d.field, 'default')
  FROM
     test t
  LEFT JOIN
     detail d ON t.id = d.item

另外，您可以使用多列NULL通过COALESCE函数检查它们。例如：

mysql> SELECT COALESCE(NULL, 1, NULL);
        -> 1
mysql> SELECT COALESCE(0, 1, NULL);
        -> 0
mysql> SELECT COALESCE(NULL, NULL, NULL);
        -> NULL