CROSS JOIN vs INNER JOIN in SQL

PriceCheaperton picture PriceCheaperton · Jul 20, 2013 · Viewed 376.9k times · Source

What is the difference between CROSS JOIN and INNER JOIN?

CROSS JOIN:

SELECT 
    Movies.CustomerID, Movies.Movie, Customers.Age, 
    Customers.Gender, Customers.[Education Level], 
    Customers.[Internet Connection], Customers.[Marital Status], 
FROM   
    Customers 
CROSS JOIN 
    Movies

INNER JOIN:

SELECT 
    Movies.CustomerID, Movies.Movie, Customers.Age, 
    Customers.Gender, Customers.[Education Level], 
    Customers.[Internet Connection], Customers.[Marital Status]
FROM   
    Customers 
INNER JOIN 
    Movies ON Customers.CustomerID = Movies.CustomerID

Which one is better and why would I use either one?

Answer

Sarath Avanavu picture Sarath Avanavu · Feb 15, 2014

Here is the best example of Cross Join and Inner Join.

Consider the following tables

TABLE : Teacher

x------------------------x
| TchrId   | TeacherName | 
x----------|-------------x
|    T1    |    Mary     |
|    T2    |    Jim      |
x------------------------x

TABLE : Student

x--------------------------------------x
|  StudId  |    TchrId   | StudentName | 
x----------|-------------|-------------x            
|    S1    |     T1      |    Vineeth  |
|    S2    |     T1      |    Unni     |
x--------------------------------------x

1. INNER JOIN

Inner join selects the rows that satisfies both the table.

Consider we need to find the teachers who are class teachers and their corresponding students. In that condition, we need to apply JOIN or INNER JOIN and will

enter image description here

Query

SELECT T.TchrId,T.TeacherName,S.StudentName 
FROM #Teacher T
INNER JOIN #Student S ON T.TchrId = S.TchrId

Result

x--------------------------------------x
|  TchrId  | TeacherName | StudentName | 
x----------|-------------|-------------x            
|    T1    |     Mary    |    Vineeth  |
|    T1    |     Mary    |    Unni     |
x--------------------------------------x

2. CROSS JOIN

Cross join selects the all the rows from the first table and all the rows from second table and shows as Cartesian product ie, with all possibilities

Consider we need to find all the teachers in the school and students irrespective of class teachers, we need to apply CROSS JOIN.

enter image description here

Query

SELECT T.TchrId,T.TeacherName,S.StudentName 
FROM #Teacher T
CROSS JOIN #Student S 

Result

x--------------------------------------x
|  TchrId  | TeacherName | StudentName | 
x----------|-------------|-------------x            
|    T2    |     Jim     |    Vineeth  |
|    T2    |     Jim     |    Unni     |
|    T1    |     Mary    |    Vineeth  |
|    T1    |     Mary    |    Unni     |
x--------------------------------------x