无论使用哪一种RDBMS，都需要使用到其中的一些日期转换函数，在使用MySQL的AddDate函数时，遇到了点小问题，稍作记录。
root@localhost:mysql3376.sock [(none)]>select now() , addtime(now(),'33 0:0:1') ; +---------------------+---------------------------+ | now() | addtime(now(),'33 0:0:1') | +---------------------+---------------------------+ | 2017-02-08 11:23:50 | 2017-03-13 11:23:51 | +---------------------+---------------------------+ 1 row in set (0.00 sec) root@localhost:mysql3376.sock [(none)]>select now() , addtime(now(),'34 0:0:1') ; +---------------------+---------------------------+ | now() | addtime(now(),'34 0:0:1') | +---------------------+---------------------------+ | 2017-02-08 11:23:54 | 2017-03-14 11:23:55 | +---------------------+---------------------------+ 1 row in set (0.00 sec) root@localhost:mysql3376.sock [(none)]>select now() , addtime(now(),'35 0:0:1') ; +---------------------+---------------------------+ | now() | addtime(now(),'35 0:0:1') | +---------------------+---------------------------+ | 2017-02-08 11:24:01 | 2017-03-15 10:24:00 | +---------------------+---------------------------+ 1 row in set, 1 warning (0.00 sec) root@localhost:mysql3376.sock [(none)]>select now() , addtime(now(),'36 0:0:1') ; +---------------------+---------------------------+ | now() | addtime(now(),'36 0:0:1') | +---------------------+---------------------------+ | 2017-02-08 11:24:05 | 2017-03-15 10:24:04 | +---------------------+---------------------------+ 1 row in set, 1 warning (0.00 sec)
仔细一看上文的4条SQL语句，第一句和第二句的结果是结果是正确的，但第三句和第四句的结果并不是期望的结果。
再仔细一看，第三句和第四句相比于第一句和第二句多输出了一个“1 warning”。来看看warning是提示了什么
root@localhost:mysql3376.sock [(none)]>show warnings ; +---------+------+--------------------------------------------+ | Level | Code | Message | +---------+------+--------------------------------------------+ | Warning | 1292 | Truncated incorrect time value: '36 0:0:1' | +---------+------+--------------------------------------------+ 1 row in set (0.00 sec)
warning提示“截断不正确的时间值”。为什么这个时间值不正确呢？
MySQL官方文档中有说明time值格式的取值说明：
MySQL recognizes TIME values in these formats: • As a string in 'D HH:MM:SS' format. You can also use one of the following “relaxed”syntaxes: 'HH:MM:SS', 'HH:MM', 'D HH:MM', 'D HH', or 'SS'. Here D represents days and can have a value from 0 to 34. • As a string with no delimiters in 'HHMMSS' format, provided that it makes sense as a time. For example, '101112' is understood as '10:11:12', but '109712' is illegal (it has a nonsensical minute part) and becomes '00:00:00'. • As a number in HHMMSS format, provided that it makes sense as a time. For example, 101112 is understood as '10:11:12'. The following alternative formats are also understood: SS, MMSS, or HHMMSS.
总结：
一、虽然MySQL的官方文档说明了time value中“D”的值只能是从0~34，但没有具体说明原因；
二、遇到问题，需要仔细查看输出内容，我只是留意到了结果有误，但并没有注意到有warning，还好有高人指点；
以上，如有错谬，请不吝指正。
