Getting data that is difficult connected with the input data, SQL Query - mysql

Question

Firstly, want to ask you (so my question would be more clear for you) if there an online database diagram tool (?) in which I can input my sql code and it will draw diagramm (with tables and their relationships) for me.
I found this question and tried some tools, but most of them can only create tables and relationships between them, but not allow to import my sql.
(2)
I need to get all classes connected with Teacher.
There are two types of connection: teacher may be the form-master or teacher may taugh this class.
How to get all classes which are connected to teacher.
(3) 1 more additional question: Am I right with constraints?
SQL Scheme (scheme on sqlfiddle.com):
CREATE TABLE `Subject` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`name` VARCHAR(50) NOT NULL DEFAULT '0',
PRIMARY KEY (`id`)
);
CREATE TABLE `Teacher` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`firstname` VARCHAR(50) NOT NULL,
`midname` VARCHAR(50) NOT NULL,
`lastname` VARCHAR(50) NOT NULL,
`address` VARCHAR(100) NOT NULL,
`birthdate` DATE NOT NULL,
PRIMARY KEY (`id`)
);
CREATE TABLE `Class` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`digit` INT(11) NOT NULL,
`char` VARCHAR(50) NOT NULL,
`teacher_id` INT(11) NOT NULL,
PRIMARY KEY (`id`),
INDEX `FK_Class_Teacher` (`teacher_id`),
CONSTRAINT `FK_Class_Teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`)
);
CREATE TABLE `ClassVsSubject` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`subject_id` INT(11) NOT NULL,
`class_id` INT(11) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE INDEX `subject_id_class_id` (`subject_id`, `class_id`),
INDEX `FK_ClassVsSubject_Class` (`class_id`),
CONSTRAINT `FK_ClassVsSubject_Class` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`),
CONSTRAINT `FK_ClassVsSubject_Subject` FOREIGN KEY (`subject_id`) REFERENCES `subject` (`id`)
);
CREATE TABLE `TeacherVsSubject` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`teacher_id` INT(11) NOT NULL,
`subject_id` INT(11) NOT NULL,
PRIMARY KEY (`id`),
INDEX `FK_TeacherVsSubject_Teacher` (`teacher_id`),
INDEX `FK_TeacherVsSubject_Subject` (`subject_id`),
CONSTRAINT `FK_TeacherVsSubject_Teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`),
CONSTRAINT `FK_TeacherVsSubject_Subject` FOREIGN KEY (`subject_id`) REFERENCES `subject` (`id`)
);

Answer 1

For your second question, use something like:
select *
from Class
where teacher_id = 1
union
select Class.*
from Class
join ClassVsSubject on Class.id = ClassVsSubject.class_id
join TeacherVsSubject on ClassVsSubject.subject_id = TeacherVsSubject.subject_id
where TeacherVsSubject.teacher_id = 1
As you did not state which teacher is the starting point, I used that with id 1.
Please note that union gets rid of duplicate entries, there is no need to treat this separately.
For your data model (question 3), I think one gotcha is that you need different entries in the subject table for different classes and the same subject like 'Maths' or 'English', otherwise, the joins will not work:
Let's say subject "English" has id 1. Now, if the class with id 1 and the class with id 2 both have this subject, you would have the following data in ClassVsSubject:
class_id subject_id
1 1
2 1
How do you want to enter into table TeacherVsSubject that the teacher with id 5 teaches English in class 1, but not in class 2? You cannot! If you enter a record with subject_id 1 and teacher_id 5, this would apply to all classes.
To remedy that, you could make one table from the two ClassVsSubject and TeacherVsSubject. This would be like this:
CREATE TABLE `ClassVsSubjectVsTeacher` (
`id` INT(11) NOT NULL AUTO_INCREMENT,
`subject_id` INT(11) NOT NULL,
`class_id` INT(11) NOT NULL,
`teacher_id` INT(11) NOT NULL,
PRIMARY KEY (`id`),
UNIQUE INDEX `subject_id_class_id` (`subject_id`, `class_id`, `teacher_id`),
INDEX `FK_ClassVsSubjectVsTeacher_Class` (`class_id`),
INDEX `FK_ClassVsSubjectVsTeacher_Teacher` (`teacher_id`),
INDEX `FK_ClassVsSubjectVsTeacher_Subject` (`subject_id`),
CONSTRAINT `FK_ClassVsSubjectVsTeacher_Class` FOREIGN KEY (`class_id`) REFERENCES `class` (`id`),
CONSTRAINT `FK_ClassVsSubjectVsTeacher_Teacher` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`),
CONSTRAINT `FK_ClassVsSubjectVsTeacher_Subject` FOREIGN KEY (`subject_id`) REFERENCES `subject` (`id`)
);
For the data model, you would not need the id column here, the primary key could be a combined key across all three foreign keys. But - depending on client software - it may be useful to have it. E. g. some OR mapping tools require a single column primary key.

Answer 2

Try this: select * from Class where teacher_id = some_id_from_Teacher_table