Question

我有一张这样的表：

<table border="1" cellspacing="0" cellpadding="5">
<tr><td>ID</td><td>status</td><td>start_date</td><td>end_date</td></tr>
<tr><td>1</td><td>A</td><td>2015-01-01</td><td>2015-12-31</td></tr>
<tr><td>1</td><td>B</td><td>2016-01-01</td><td>NULL</td></tr>
<tr><td>2</td><td>C</td><td>NULL</td><td>2016-12-31</td></tr>
<tr><td>3</td><td>D</td><td>NULL</td><td>NULL</td></tr>
</table>

我必须在MySQL中执行一个触发器来更新主表的最后状态。

要执行此操作，我必须根据ID和status之间的时间段选择最后start_date的{{1}}，如果不知道则可以为NULL。< / p>

我写这个SQL语句测试：

end_date

是否有缩短方法来执行此操作？

实际上我想要一个能够捕获每个案例的WHERE子句（a = start_date，b = end_date）：

SELECT * FROM My_Table AS T WHERE T.start_date Is Null AND T.end_date In(SELECT MAX(end_date) FROM My_Table WHERE ID = T.ID) OR T.start_date In(SELECT MAX(start_date) FROM My_Table WHERE ID = T.ID) AND T.end_date Is Null OR T.start_date In(SELECT MAX(start_date) FROM My_Table WHERE ID = T.ID) AND T.end_date In(SELECT MAX(end_date) FROM My_Table WHERE ID = T.ID );

Answer 1

这是一个稍微简化的查询，它返回与您问题中的查询完全相同的结果集。它只使用两个子查询而不是四个（没有重复的子查询）：

SELECT *
FROM My_Table AS T
WHERE 
  (T.start_date Is Null OR T.start_date = (SELECT MAX(start_date) FROM My_Table WHERE ID = T.ID)) AND
  (T.end_date Is Null OR T.end_date = (SELECT MAX(end_date) FROM My_Table WHERE ID = T.ID)) AND 
  (T.start_date Is NOT Null OR T.end_date Is NOT Null)
;

请注意，它不包含a = NULL AND b = NULL案例。要包括它，必须删除最后一个AND子句，即：

SELECT *
FROM My_Table AS T
WHERE 
  (T.start_date Is Null OR T.start_date = (SELECT MAX(start_date) FROM My_Table WHERE ID = T.ID)) AND 
  (T.end_date Is Null OR T.end_date = (SELECT MAX(end_date) FROM My_Table WHERE ID = T.ID)) 
;

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