从不同的日期时间字段中选择最新的日期时间

Question

不同的表中有不同的日期时间字段，我必须选择所有日期时间字段中的最新日期时间字段。的表是：

Question 
-------- 
askedby 
questionType 
lastupdated 

Document 
---------- 
submittedBy 
documentType 
lastupdated 

Instruction 
----------- 
submittedBy 
instructionType 
lastupdated 

试图启动与

SELECT Question.lastupdated,Document.lastupdated,Instruction.lastupdated 
FROM Question,Document,Instruction 
WHERE lastupdated=(select max(lastupdated)) 
ORDER BY lastupdated DESC 

如果LASTUPDATED表中的值是：

Question table: 2015-06-28 07:00:00 
Document table: 2015-06-28 07:30:00 
Instruction table: 2015-06-28 08:00:00 

然后从Instruction表中的值应选择，像2015-06-28 08:00:00

在这里，我必须选择最新的拉斯来自三个数据库的tupdated列。我是SQL查询的新手，不确定如何开始。

Answer 1

由于它看起来像只想从三个分离集合中提取最后一个日期，所以一个简单的选项将对每个表格使用查询，并使用union all从三个结果中制作一个集合并将max应用于该集合，像这样：

SELECT MAX(lastupdated) AS max_lastupdated 
FROM 
(
    SELECT MAX(lastupdated) lastupdated FROM Question 
    UNION ALL 
    SELECT MAX(lastupdated) lastupdated FROM Document 
    UNION ALL 
    SELECT MAX(lastupdated) lastupdated FROM Instruction 
) A 

其实你如果索引支持甚至都不需要选择三个select语句的最大值，刚开LASTUPDATED应该够用了，虽然上面的查询可能会更快：

SELECT MAX(lastupdated) AS max_lastupdated 
FROM 
(
    SELECT lastupdated FROM Question 
    UNION ALL 
    SELECT lastupdated FROM Document 
    UNION ALL 
    SELECT lastupdated FROM Instruction 
) A 

Sample SQL Fiddle