如果未找到匹配项，则返回某个值的条件答案

【问题标题】：where condition to return some value if not match found如果未找到匹配项，则返回某个值的条件
【发布时间】：2026-01-13 11:05:01
【问题描述】：

SELECT
  PROJECT_ESTIMATES_ID,
  PIPELINE_STEP_ID
FROM
  PROJECT_ESTIMATES
WHERE
  PROJECT_BASIC_INFORMATION_ID='29ab9760-c75b-4479-882c-bc84426d55ac';

这给出了以下输出

和

通过将 where 条件与其他类一起使用，

SELECT
  PE.PROJECT_ESTIMATES_ID,
  MPS.STEP_DESCRIPTION
FROM
  PROJECT_ESTIMATES PE,
  MST_PIPELINE_STEPS MPS 
WHERE
  PE.`PROJECT_BASIC_INFORMATION_ID` ='29ab9760-c75b-4479-882c-bc84426d55ac' 
  AND PE.`TENANT_ID`='{0559cdcb-c63b-4c81-be91-b78Tenant1000'
  AND PE.PIPELINE_STEP_ID=MPS.PIPELINE_STEP_ID
ORDER BY
  PE.MODIFIED_DATE DESC;

给出以下输出。

我想将第一个记录的 step_description 设为 0，它的 PROJECT_ESTIMATES_ID 值为 046a190e-a895-4ce2-bb10-7a583d648b99.ie，下图显示了所需的输出。

【问题讨论】：

标签： mysql join inner-join where outer-join

【解决方案1】：

如果我没有误解你的问题，这将有效：

SELECT 
PE.PROJECT_ESTIMATES_ID,
IF(
PE.PROJECT_ESTIMATES_ID = '046a190e-a895-4ce2-bb10-7a583d648b99', 
0,
PE.PROJECT_ESTIMATES_ID) AS STEP_DESCRIPTION
FROM 
PROJECT_ESTIMATES PE, 
MST_PIPELINE_STEPS MPS 
WHERE PE.`PROJECT_BASIC_INFORMATION_ID` ='29ab9760-c75b-4479-882c-bc84426d55ac' 
AND PE.`TENANT_ID`='{0559cdcb-c63b-4c81-be91-b78Tenant1000' 
AND PE.PIPELINE_STEP_ID=MPS.PIPELINE_STEP_ID 
ORDER BY PE.MODIFIED_DATE DESC

当然，除非你的意思是你想要零而不是空白或 null，在这种情况下，这就是你想要的：

SELECT 
PE.PROJECT_ESTIMATES_ID
IF(
LENGTH
(ISNULL(
PE.PROJECT_ESTIMATES_ID,'')) > 0,
PE.PROJECT_ESTIMATES_ID, 0) AS STEP_DESCRIPTION
FROM PROJECT_ESTIMATES PE, 
     MST_PIPELINE_STEPS MPS 
WHERE PE.`PROJECT_BASIC_INFORMATION_ID` ='29ab9760-c75b-4479-882c-bc84426d55ac' 
AND PE.`TENANT_ID`='{0559cdcb-c63b-4c81-be91-b78Tenant1000' 
AND PE.PIPELINE_STEP_ID=MPS.PIPELINE_STEP_ID 
ORDER BY PE.MODIFIED_DATE DESC

【讨论】：

hi.sorry for your misunderstanding.please see the added image (desired output) 以上。
我的答案与您所寻找的相差无几，我已根据您的要求对其进行了修改。
hi @alan 在执行 sql.Query 时出错：SELECT PE.PROJECT_ESTIMATES_ID IF(LENGTH(ISNULL(PE.PROJECT_ESTIMATES_ID,'')) > 0,PE.PROJECT_ESTIMATES_ID, 0) AS STEP_DESCRIPTION.. . 错误代码：1064

【解决方案2】：

这个查询满足我的要求。

SELECT PE.PROJECT_ESTIMATES_ID, MPS.STEP_DESCRIPTION
FROM PROJECT_ESTIMATES PE
LEFT JOIN MST_PIPELINE_STEPS MPS
ON PE.PIPELINE_STEP_ID=MPS.PIPELINE_STEP_ID WHERE
PE.`PROJECT_BASIC_INFORMATION_ID` ='29ab9760-c75b-4479-882c-bc84426d55ac'
AND PE.`TENANT_ID`='{0559cdcb-c63b-4c81-be91-b78Tenant1000' 
ORDER BY PE.MODIFIED_DATE DESC ;

【讨论】：