Mysql – How to insert values into a table from two different tables

MySQLrelational-theory

I have three tables

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)

If I was given teacher's name (david for example) and student_id (7 for example) and asked to insert the teacher_id into the classroom table based on the id in the teachers table, I would do :

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

Now, what if I was not given the student's id directly and given only the name of the student? Suppose I was given teacher's name 'david' and student's name 'sam'. How do I get the teacher_id from teachers table and also student_id from the students table and insert both into the classroom table based on their respective names?

Best Answer

You would write the query like this

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';

Be careful. This is a Cartesian product. Another way to approach this is

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);
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