Inner Join vs Outer Join

What is Join?
An SQL Join is used to combine data from two or more tables, based on a common field between them. For example, consider the following two tables.

Student Table

EnrollNo	StudentName	Address
1001	geek1	geeksquiz1
1002	geek2	geeksquiz2
1003	geek3	geeksquiz3
1004	geek4	geeksquiz4

StudentCourse Table

CourseID	EnrollNo
1	1001
2	1001
3	1001
1	1002
2	1003

Following is join query that shows names of students enrolled in different courseIDs.

SELECT StudentCourse.CourseID,Student.StudentName
FROM Student
INNER JOIN StudentCourse 
ON StudentCourse.EnrollNo = Student.EnrollNo
ORDER BY StudentCourse.CourseID

Note:  INNER is optional above.  Simple JOIN is also considered as INNER JOIN

The above query would produce following result.

CourseID	StudentName
1	geek1
1	geek2
2	geek1
2	geek3
3	geek1

What is the difference between inner join and outer join?

Outer Join is of 3 types
1) Left outer join
2) Right outer join
3) Full Join

1) Left outer join returns all rows of table on left side of join. The rows for which there is no matching row on right side, result contains NULL in the right side.

SELECT Student.StudentName,
       StudentCourse.CourseID
FROM Student
LEFT OUTER JOIN StudentCourse 
ON StudentCourse.EnrollNo = Student.EnrollNo
ORDER BY StudentCourse.CourseID

Note: OUTER is optional above. Simple LEFT JOIN is also considered as LEFT OUTER JOIN

StudentName	CourseID
geek4	NULL
geek2	1
geek1	1
geek1	2
geek3	2
geek1	3

2) Right Outer Join is similar to Left Outer Join (Right replaces Left everywhere)

3) Full Outer Join Contains results of both Left and Right outer joins.

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