Primary keys in sql

Question

I have here some problems undersanding how this works. So I have 2 tables Students and Enrolled like this:

CREATE TABLE Students 
 (sid CHAR(20), 
 name CHAR(50), 
 email CHAR(30), 
 age INTEGER, 
 gr INTEGER) 

CREATE TABLE Enrolled 
 (sid CHAR(20), 
 cid CHAR(20), 
 grade CHAR(2), 
PRIMARY KEY (sid,cid)) 

So I don't understand this particulary row PRIMARY KEY (sid,cid) Can someone explain to me how it works? I have to specify that I have another table Courses from where the cid.

Is is equivalent saying like this:

CREATE TABLE Enrolled
(sid CHAR(20) foreign key references Students(sid),
cid CHAR(20) foreign key references Courses(cid)
)

Answer 1

PRIMARY KEY means both UNIQUE and NOT NULL .

If you want sid and cid to also be FOREIGN KEY you have to specify that separately.

Having two fields for a primary key is often used for tables that are the physical representation of a many-to-many relation. In your database design diagram you would have STUDENT and COURSE as entities and ENROLLMENT as a many-to-many relationship between them.

In the physical database diagram many-to-many relationships are modelled as tables, often with a composite PRIMARY KEY and with FOREIGN KEY constraints to the entity tables.

Answer 2

PRIMARY KEY（sid，cid）表示复合主键..这些字段的组合应该是唯一的。

Answer 3

A PRIMARY KEY is used to identify a table. A field or column that is defined as PRIMARY KEY will contains different values on each row in that table, and is mandatory to have a value (so, PRIMARY KEY is equivalent to UNIQUE and NOT NULL ).

PRIMARY KEY can be a single field, or multiple fields, but it always satisfy that "each row will have a different PRIMARY KEY ".

If you declare as PRIMARY KEY a combination of 2 columns, you will be able to have this for example:

CREATE TABLE Enrolled 
 (sid CHAR(20), 
 cid CHAR(20), 
 grade CHAR(2), 
PRIMARY KEY (sid,cid)) --PRIMARY KEY with combination of 2 columns

---

sid   |   cid   |   grade
1          1         XX
1          2         XX
2          1         XX
2          2         XX
2          3         XX

In this example, you can see that the column sid or the column cid has repeated values individually, but there isn't a combination of (sid, cid) that was repeated.

Like a PRIMARY KEY is used to identify a row in a table, when we want to relate two tables we can define a FOREIGN KEY in one table to link this one with the other table.

Your case is that the ENROLLED table is identified by a composite PRIMARY KEY to represent a many-to-many relationship. That is the way to say that:

• A student can be enrolled in many courses.
• A course can have enrolled many students.

Note*: Is a best practice to define the PRIMARY KEYS as numeric values, such integer , bigint , etc. because it is better to improve the indexes performance (all PRIMARY KEYS have defined inherently an INDEX , and they are faster working with "numeric" values than working with "string" values).