如何为不返回行的查询设置默认行？

Question

如果表中没有行，我需要知道如何返回默认行。什么是最好的方法来做到这一点？我只是从这个特定的表中返回一列来获得它的价值。

编辑：这将是SQL Server。

Answer 1

在Oracle：

SELECT val 
FROM myTable 
UNION ALL 
SELECT 'DEFAULT' 
FROM dual 
WHERE NOT EXISTS (SELECT * FROM myTable) 

或者：

SELECT NVL(MIN(val), 'DEFAULT') 
FROM myTable 

Answer 2

你想返回一整行吗？默认行是否需要具有默认值或者它是否为空行？您是否希望默认行具有与所讨论的表相同的列结构？

根据您的要求，你可能会做这样的事情：

1）运行查询，并放在一个临时表结果（或表变量） 2）检查是否临时表有结果 3）如果没有，通过执行与此类似（SQL Server中的select语句）返回一个空行：

select '' as columnA, '' as columnB, '' as columnC from #tempTable 

凡columnA，columnB和columnC是实际的列名。

Answer 3

我想通了，也应该对其他系统的工作了。这是WW的答案的变体。

select rate 
from d_payment_index 
where fy = 2007 
    and payment_year = 2008 
    and program_id = 18 
union 
select 0 as rate 
from d_payment_index 
where not exists(select rate 
        from d_payment_index 
        where fy = 2007 
        and payment_year = 2008 
        and program_id = 18) 

Answer 4

这将是运行两次消除选择查询，并获得更好的性能：使用左

Declare @rate int 

select 
    @rate = rate 
from 
    d_payment_index 
where 
    fy = 2007 
    and payment_year = 2008 
    and program_id = 18 

IF @@rowcount = 0 
    Set @rate = 0 

Select @rate 'rate' 

Answer 5

一个表扫描方法从默认加入到实际值：

CREATE TABLE [stackoverflow-285666] (k int, val varchar(255)) 

INSERT INTO [stackoverflow-285666] 
VALUES (1, '1-1') 
INSERT INTO [stackoverflow-285666] 
VALUES (1, '1-2') 
INSERT INTO [stackoverflow-285666] 
VALUES (1, '1-3') 
INSERT INTO [stackoverflow-285666] 
VALUES (2, '2-1') 
INSERT INTO [stackoverflow-285666] 
VALUES (2, '2-2') 

DECLARE @k AS int 
SET @k = 0 

WHILE @k < 3 
    BEGIN 
     SELECT @k AS k 
       ,COALESCE(ActualValue, DefaultValue) AS [Value] 
     FROM (
       SELECT 'DefaultValue' AS DefaultValue 
       ) AS Defaults 
     LEFT JOIN (
        SELECT val AS ActualValue 
        FROM  [stackoverflow-285666] 
        WHERE k = @k 
       ) AS [Values] 
       ON 1 = 1 

     SET @k = @k + 1 
    END 

DROP TABLE [stackoverflow-285666] 

给出输出：

k   Value 
----------- ------------ 
0   DefaultValue 

k   Value 
----------- ------------ 
1   1-1 
1   1-2 
1   1-3 

k   Value 
----------- ------------ 
2   2-1 
2   2-2 

Answer 6

如果你的基本查询预计只返回一行，那么你可以使用这一招：如果NVL是SQL Server的正确功能

select NVL(MIN(rate), 0) AS rate 
from d_payment_index 
where fy = 2007 
    and payment_year = 2008 
    and program_id = 18 

（甲骨文代码，不知道。）

Answer 7

如何：

SELECT DEF.Rate, ACTUAL.Rate, COALESCE(ACTUAL.Rate, DEF.Rate) AS UseThisRate 
FROM 
    (SELECT 0) DEF (Rate) -- This is your default rate 
LEFT JOIN (
    select rate 
    from d_payment_index 
    --WHERE 1=2 -- Uncomment this line to simulate a missing value 

    --...HERE IF YOUR ACTUAL WHERE CLAUSE. Removed for testing purposes... 
    --where fy = 2007 
    -- and payment_year = 2008 
    -- and program_id = 18 
) ACTUAL (Rate) ON 1=1 

结果

有效利率时使用

Rate  Rate  UseThisRate 
----------- ----------- ----------- 
0   1   1 

违约率

Rate  Rate  UseThisRate 
----------- ----------- ----------- 
0   NULL  0 

测试DDL

CREATE TABLE d_payment_index (rate int NOT NULL) 
INSERT INTO d_payment_index VALUES (1) 

Answer 8

插入您的默认值到一个表变量，然后与你的实际表匹配更新此tableVar的单行。如果找到一行，tableVar将被更新;如果不是，则保留默认值。返回表变量。

---=== The table & its data 
    CREATE TABLE dbo.Rates (
     PkId int, 
     name varchar(10), 
     rate decimal(10,2) 
    ) 
    INSERT INTO dbo.Rates(PkId, name, rate) VALUES (1, 'Schedule 1', 0.1) 
    INSERT INTO dbo.Rates(PkId, name, rate) VALUES (2, 'Schedule 2', 0.2) 

这里的解决方案：

---=== The solution 
CREATE PROCEDURE dbo.GetRate 
    @PkId int 
AS 
BEGIN 
    DECLARE @tempTable TABLE (
    PkId int, 
    name varchar(10), 
    rate decimal(10,2) 
) 

--- [1] Insert default values into @tempTable. PkId=0 is dummy value 
INSERT INTO @tempTable(PkId, name, rate) VALUES (0, 'DEFAULT', 0.00) 

--- [2] Update the single row in @tempTable with the actual value. 
---  This only happens if a match is found 
UPDATE @tempTable 
    SET t.PkId=x.PkId, t.name=x.name, t.rate = x.rate 
    FROM @tempTable t INNER JOIN dbo.Rates x 
    ON t.PkId = 0 
    WHERE x.PkId = @PkId 

SELECT * FROM @tempTable 
END 

测试代码：

EXEC dbo.GetRate @PkId=1  --- returns values for PkId=1 
EXEC dbo.GetRate @PkId=12314 --- returns default values