如何在mysql中給子查詢默認值？

Question

在查詢中，我可以給子查詢一些默認值嗎？

我的子查詢之一可能沒有任何值。在這種情況下，它返回null作為整個結果。 即使子查詢之一為null，也可以分配一些默認值嗎？

select 
    t1 . *, t2.speed, t3.Accelerometerx
from
    (SELECT 
        *
    FROM
        `table1`
    ) as t1,
    (SELECT 
       *
    FROM
        table1)
    as t2

Answer 1

檢查nvl函數，它應該可以解決您的問題。

Answer 2

使用情況/如果：

CASE case_value
    WHEN when_value THEN statement_list
    [WHEN when_value THEN statement_list] ...
    [ELSE statement_list]
END CASE

示例，而不是：

where
    `vehicle_tripdetails`.`Param_ID` = '131'
        and trip_id = 127) as t1,
(SELECT 
    time_stamp, `vehicle_tripdetails`.`Param_Data` as speed
FROM
    `vehicle_tripdetails`
where
    `vehicle_tripdetails`.`Param_ID` = '13'
        and trip_id = 127) as t2,

您可以使用：

where
    `vehicle_tripdetails`.`Param_ID` = '131'
        and trip_id = 127) as t1,
CASE 
  WHEN (SELECT  time_stamp, `vehicle_tripdetails`.`Param_Data` as speed FROM
    `vehicle_tripdetails` where
    `vehicle_tripdetails`.`Param_ID` = '13'
        and trip_id = 127) 
  IS NOT NULL 
THEN (SELECT  time_stamp, `vehicle_tripdetails`.`Param_Data` as speed FROM
    `vehicle_tripdetails` where
    `vehicle_tripdetails`.`Param_ID` = '13'
        and trip_id = 127) 
ELSE
   'default value'
END CASE
AS t2

Answer 3

如下所示在您的select子句中使用ISNull

select    t1.*,
          CASE ISNULL(t2.speed) WHEN 1 THEN 0 ELSE t2.speed END,
          CASE ISNULL(t3.xyz) WHEN 1 THEN 0 ELSE t3.xyz END

此處的默認值為0。如果您未使用*選擇所有記錄，則此功能有效