我有一个 Microsoft SQL Server 2008 查询,它使用左外连接从三个表中返回数据。很多时候,第二个和第三个表中没有数据,所以我得到一个空值,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,我希望 Quantity 和 RegularPrice 默认为零。
这很容易
IsNull(FieldName, 0)
或更完整地说:
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'
对于 MySQL
或 SQLite
,正确的关键字是 IFNULL
(不是 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'
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
此外,您可以使用多个列通过 COALESCE 函数检查它们的 NULL
。例如:
mysql> SELECT COALESCE(NULL, 1, NULL);
-> 1
mysql> SELECT COALESCE(0, 1, NULL);
-> 0
mysql> SELECT COALESCE(NULL, NULL, NULL);
-> NULL
对于 Oracle,您可以使用:
NVL(columnName,deafultValue)
:- NVL 用于将空值转换为查询输出中的默认值。例如。如果你想用 'NA' 替换空值,那么使用类似这样的东西。
SELECT NVL(columnName,'NA') FROM tableName
IFNULL
。