如何将值从两个不同的表插入一个表？

Question

我有三张桌子

students table 
------------------------------------  
id(PK, A_I)  |  student_name | nationality

teachers table
------------------------------------
id(PK, A_I)  |  teacher_name |  email

classroom table
----------------------
id(PK, A_I)   | date   | teacher_id(FK to teachers.id)  |  student_id(FK to students.id)

如果我得到了老师的名字（david例如）和student_id数据（7例如），并要求插入teacher_id到classroom基于表id的teachers表，我会做：

insert into classroom (date, teacher_id, student_id)
select '2014-07-08', id, 7
from teachers
where teacher_name = 'david';

现在，如果我没有直接获得学生的 id 而只给我学生的名字怎么办？假设我得到了老师的名字“大卫”和学生的名字“山姆”。我如何获取teacher_id从teachers表中，也student_id从students表和插入双双进入classroom表基于各自的名称？

Answer 1

你会写这样的查询

insert into classroom (date, teacher_id, student_id)
select '2014-07-08', t.id, s.id
from teachers t,students s
where t.teacher_name = 'david'
and s.student_name = 'sam';

当心。这是笛卡尔积。解决这个问题的另一种方法是

select teacher_id into @tid from teachers where teacher_name = 'david';
select student_id into @sid from students where student_name = 'sam';
insert into classroom (date, teacher_id, student_id) values ('2014-07-08',@tid,@sid);

Answer 2

最简单的方法是使用子查询：

 INSERT INTO classroom(teacher_id,student_id)
 VALUES ((SELECT id FROM students WHERE s_name='sam'),
 (SELECT id FROM teacher WHERE t_name='david'));