Sometime in a table we need two or more column to uniquely identify each row . Composite key is the combination of two or more Primary key . Primary key or candidate key come from the composite key .

Syntax in SQL :

create table table_name col_name1 data_type1 , col_name1 data_type2 , ???? Primary key ( col_name1 , col_name2 , ?.));

Syntax in MySQL :

create table table_name col_name1 data_type1 , col_name1 data_type2 , ......... Primary key ( col_name1 , col_name2));

Example :
Table name : student

Name        Branch        Section         Roll_Id
Mukesh        CS               A            15
Ravi          EE               A             2
Ayush         ME               B            15

Second table : student_report

Branch      Section           Roll_Id        Grade     Semester    Attendance 
CS              A               15            A++       1st           90
EE              A                2            A+        2nd           70
CS              B               15            AA        1st           85
EE              A                2            A+        1st           70 
CS              A               15            AA        2nd           85

Note :
class , section , and roll_id combination is primary key in student table .
class , section , and roll_id combination is foreign key in student_report table and this act as a primary key in student table .
class , section , and roll_id of student table must be same as student_report table .
Query :

select a.name , a.class , a.section , a.roll_id , b.grade , b.semester from student a , student_report b where a.class = b.class and a.section = b.section and a.roll_id = b.roll_id ;

Output :

Name      Class      Section      Roll_Id          Grade           Semester
Ravi       EE           A           2                A+               2nd
Ayush      CS           B          15               AA                1st 
Ravi       EE           A           2               A+                1st
Mukesh     CS           A          15               AA                2nd
Mukesh     CS           A          15               A++               1st