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select distinct students.name from students join documents on 
students.user_id=documents.user_id where documents.user_id is No

------+| 1 | f1 | l1 |+

---------+|学生|学号|学生|姓|+

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------+一行（0.02秒）

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mysql> create table students (student_number int, student_first_name char(25), student_Last_name char(25));

查询正常，0行受影响（0.34秒）

mysql> create table documents (student_number int, document_name char(25), approved bool);

查询正常，0行受影响（0.32秒）

mysql> insert into students values (1,"F1","L1"),(2,"F2","L2"),(3,"F3","L3");

查询正常，3行受影响（0.19秒）记录：3个重复：0警告：0

mysql> insert into documents values (1,"D1",0),(1,"D2",1),(3,"D3",1);

查询正常，3行受影响（0.16秒）记录：3个重复：0个警告：0

mysql> select * from students where student_number in (select student_number from documents where !approved);

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