如何在sql的where条件中定义if子句?

通常在SQL中使用布尔逻辑处理这个问题。所以你可以写:

WHERE  StartDate = @reportMonth
AND..
AND ...
AND (@siteId <=0 OR  sites.SiteId = @siteId)

所以如果 @siteId 不大于0它忽略 OR 子句,但如果是,则它将评估 sites.SiteId 条款。通常对于存储过程,我会将默认值设置为空并检查是否为空:

CREATE PROCEDURE [dbo].[usp_GetStatus]

@siteId INT = NULL,
........
from sites;
 WHERE  StartDate = @reportMonth
AND..
AND ...
AND (@siteId IS NULL OR  sites.SiteId = @siteId)

我宁愿建议

CREATE PROCEDURE [dbo].[usp_GetStatus]
    @siteId INT,


    ........
    FROM sites
    WHERE StartDate = @reportMonth
      AND..
      AND ...
      AND 
sites.SiteId = case when @siteId > 0 then @siteId else sites.SiteId end