如何计算MySql中另一个表中数据\\行的条目？

Question

The database and entries are as in the figure below: Teacher table Course table

1) Show how many courses each teacher is teaching. 2) Give the name of the teacher who teaches more than two courses.

Is it necessary to use join(s) or can we perform the actions by any other means?

Thank you in advance. Any help is appriciated.

Answer 1

A join is necessary, as you are trying to receive a result based on data from two tables. To even know what courses a teacher is teaching, you must join the tables.

Imagine you were at a hackathon, and you met a friendly coder there. You remember one very distinct thing about him - we'll say he had a very distinct hair style that no one else at the hackathon had. However, that's the only thing you can remember about him. You ask your friend that was with you if he remembers your new found friend, describing him by his distinct hair style. Lucky you, he remembers! Your friend then tells you all the other details about your new found hackathon companion (eg his name, where he was from, etc.). In this case, your friend is the join. You only knew one distinct thing about the hackathon man (like a foreign key in a database), but it was distinct enough to get you the rest of the information about the man.

Putting this in context of your situation, your tables will need to have a foreign key relationship. This will allow you to link the tables together so that you can see what courses each teacher is in charge of.