[SQL] 的表关系是什么？

Question

I have really dumb question...,

for what are exactly relationships in databases?

If I selecting data from two tables for example:

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers ON Orders.CustomerID=Customers.CustomerID;

In query is exactly specified the columns of what tables will be connected to each other. Result will be same, no matter if I use foreign keys.

Maybe I am not enough good to find answer, but everywhere is explained HOW to use them, but not WHY I should use them.

PS.: Sorry for bad English, google translator did a lot. :)

Answer 1

Relationships in Databases are used to maintain connection between two entities. For example: classes table and Teachers table. a teacher can take more than one class which means there is a one to many relationship here and by separating data into two separate entities help to maintain referential integrity