--- ###一、问题来源 最近遇到一个日期比较的问题如下： ``` root@localhost:test:10:25:48>select from_unixtime(1596680255); +---------------------------+ | from_unixtime(1596680255) | +---------------------------+ | 2020-08-06 10:17:35 | +---------------------------+ 1 row in set (0.00 sec) root@localhost:test:10:25:52>select from_unixtime(1596680255,'%Y-%m-%d') < '2020-08-1'; +----------------------------------------------------+ | from_unixtime(1596680255,'%Y-%m-%d') < '2020-08-1' | +----------------------------------------------------+ | 1 | +----------------------------------------------------+ 1 row in set (2.70 sec) root@localhost:test:10:25:59>select from_unixtime(1596680255) < '2020-08-1'; +-----------------------------------------+ | from_unixtime(1596680255) < '2020-08-1' | +-----------------------------------------+ | 0 | +-----------------------------------------+ 1 row in set (2.36 sec) ``` 按理来说from_unixtime(1596680255,'%Y-%m-%d') < '2020-08-1' 应该是false才对，但是返回为true，因此怀疑为字符串比较的方式 ###二、问题验证 首先关闭gtid通过create table as 来验证一下字段类型如下： ``` create table testit951 as select from_unixtime(1596678161,'%Y-%m-%d') as "tt" create table testit952 as select from_unixtime(1596678161) as "tt" ``` 通过这种方式发现两种建表字段为varchar和datetime类型。通过源码验证可以看到如下： ``` Breakpoint 5, Arg_comparator::compare_string (this=0x7fffe40167c0) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/item_cmpfunc.cc:1672 1672 if ((res1= (*a)->val_str(&value1))) (gdb) c Continuing. Breakpoint 4, Arg_comparator::compare_datetime (this=0x7fffe40164c8) at /cdh/mysqldebug/percona-server-5.7.29-32/sql/item_cmpfunc.cc:1509 1509 THD *thd= current_thd; (gdb) c Continuing. ``` 可以看到内部通过了string和datetime两种类型来比较。因此得到证明。最后留下比较函数的位置： Item_cmpfunc.cc