MySQL-根据另一张表的列数从一张表中获取记录

Question

I have a table called students and a table called documents . Each student can have any amount of document entries in the documents table, but might also have no document entries. Some of the documents might've been approved, others not.

table 1: students , table 2 documents . student PK is user_id and document PK is document_id , and document table has user_id in it as well. document table has column approved which can contain either a Yes or a No. So these two tables are linked by user_id

How can I write a MySQL query (or even better, in Active Record style for Code Igniter) that can list all students that have at least 1 unapproved document?

Answer 1

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

Query OK, 0 rows affected (0.34 sec)

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

Query OK, 0 rows affected (0.32 sec)

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

Query OK, 3 rows affected (0.19 sec) Records: 3 Duplicates: 0 Warnings: 0

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

Query OK, 3 rows affected (0.16 sec) Records: 3 Duplicates: 0 Warnings: 0

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

+----------------+--------------------+-------------------+ | student_number | student_first_name | student_Last_name | +----------------+--------------------+-------------------+ | 1 | F1 | L1 | +----------------+--------------------+-------------------+ 1 row in set (0.02 sec)

Answer 2

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