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Inner Join vs Outer Join

  • Difficulty Level : Easy
  • Last Updated : 09 May, 2017

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

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EnrollNoStudentNameAddress
1001geek1geeksquiz1
1002geek2geeksquiz2
1003geek3geeksquiz3
1004geek4geeksquiz4

StudentCourse Table



CourseIDEnrollNo
11001
21001
31001
11002
21003

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.

CourseIDStudentName
1geek1
1geek2
2geek1
2geek3
3geek1

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

StudentNameCourseID
geek4NULL
geek21
geek11
geek12
geek32
geek13

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.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above

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