SQL 错误（从字符串转换日期 and/or 时间时转换失败）

Question

我正在尝试编写 SELECT CASE 表达式来检查列是否为 NULL。如果它不是 NULL，则 return 该列。否则 return "NA"。我在 FROM 中引用了正确的 table。不知道为什么我在从字符串错误转换日期 and/or 时间时得到转换失败。

(
SELECT
    CASE
        WHEN DateCompleted IS NOT NULL THEN dbo.WF_InstanceSteps.DateCompleted
        ELSE ''NA''
    END AS DateCompleted
FROM
    dbo.WF_InstanceSteps
WHERE
    (StepId = 6) AND
    (InstanceId = dbo.WF_Instances.InstanceId)
) AS ApprovalDate

Answer 1

这应该有效：

SELECT 
    CASE 
        WHEN wi.DateCompleted IS NOT NULL THEN wis.DateCompleted 
        ELSE 'NA' 
    END AS DateCompleted
FROM dbo.WF_Instances AS wi
    INNER JOIN dbo.WF_InstanceSteps AS wis
        ON wi.instance_id = wis.instance_id
WHERE wi.StepId = 6