从三个不同的选择最大查询中选择最大值

Question

我有这个查询获取三个不同表的最大日期记录。我可以在这些桌子上做一个工会，让他们全部返回他们各自的日期。但是，我无法找到一种方法来选择返回的这三条记录的最大值。有任何想法吗？

SELECT MAX (Date_submitted) as a 
FROM [AngelSQL] . [dbo]. [LSN_RESPONSES] , ACCOUNTS , LSN_ENTRIES , GB3_ASSOCIATIONS 
where 
LSN_RESPONSES . USER_ID = accounts . username 
and LoginName = 'admin' 
and TakenFromCourseId = 'TT-TC-TS-TD-Test-Test' 
and LSN_RESPONSES . PARENT_ID = LSN_ENTRIES .ENTRY_ID 
and LSN_RESPONSES . PARENT_ID = GB3_ASSOCIATIONS .LSN_ENTRY_ID 
union 
SELECT MAX (dateEnd) as a 
FROM [AngelSQL] . [dbo]. [qti_assessment_attempt] ,dbo . ACCOUNTS, COURSES, qti_question_containers, GB3_ASSOCIATIONS 
where accounts . PERSON_ID = qti_assessment_attempt .person_id 
and 
taken_from_course_container_id = courses. CONTAINER_ID 
and 
accounts .LoginName = 'admin' 
and COURSES . COURSE_ID= 'TT-TC-TS-TD-Test-Test' 
and qti_assessment_attempt . question_container_id = qti_question_containers. id 
and qti_question_containers . entry_ID = GB3_ASSOCIATIONS .LSN_ENTRY_ID 
union 
SELECT MAX (PostDate) as a 
FROM dbo . ACCOUNTS, DiscussionPost , DiscussionForum , GB3_ASSOCIATIONS 
where 
accounts .USERNAME = DiscussionPost. AuthorId 
and 
LoginName ='admin' 
and 
discussionpost . CourseContext = 'TT-TC-TS-TD-Test-Test' 
and 
DiscussionPost . ForumId = DiscussionForum .ForumId 
and DiscussionForum . EntryId = GB3_ASSOCIATIONS .LSN_ENTRY_ID} 

Answer 1

使用您的查询作为子查询/ CTE，像这样：

with t as (<put your query here>) 
select max(a) 
from t