有一個(gè)表，名字是main，表內(nèi)容如下：mysql>select*frommain;+------+----------+|ID|username|+------+----------+|0|user0||1|user1||2|user2|+------+----------+執(zhí)行命令返回：mys...  有一個(gè)表，名字是main，表內(nèi)容如下：mysql> select * from main;+------+----------+| ID   | username |+------+----------+|    0 | user0    ||    1 | user1    ||    2 | user2    |+------+----------+執(zhí)行命令返回：mysql> select username from main where username='user0' || (select 1 from dual where (select username from main where username='user0')='user0');+----------+| username |+----------+| user0    || user1    || user2    |+----------+為什么會(huì)有這個(gè)結(jié)果？我從來(lái)沒有在我的語(yǔ)句里面涉及user1和user2，順便說(shuō)一下，單獨(dú)執(zhí)行執(zhí)行第二句select 1 from dual where (select username from main where username='user0')='user0'結(jié)果如下：+---+| 1 |+---+| 1 |+---+  展開