SQL JOIN

As the name shows, JOIN means to combine something. In case of SQL, JOIN means "to combine two or more tables".

In SQL, JOIN clause is used to combine the records from two or more tables in a database.

Types of SQL JOIN

1. INNER JOIN
2. LEFT JOIN
3. RIGHT JOIN
4. FULL JOIN

Sample Table

EMPLOYEE

EMP_ID	EMP_NAME	CITY	SALARY	AGE
1	Angelina	Chicago	200000	30
2	Robert	Austin	300000	26
3	Christian	Denver	100000	42
4	Kristen	Washington	500000	29
5	Russell	Los angels	200000	36
6	Marry	Canada	600000	48

PROJECT

PROJECT_NO	EMP_ID	DEPARTMENT
101	1	Testing
102	2	Development
103	3	Designing
104	4	Development

1. INNER JOIN

In SQL, INNER JOIN selects records that have matching values in both tables as long as the condition is satisfied. It returns the combination of all rows from both the tables where the condition satisfies.

Syntax

1. SELECT table1.column1, table1.column2, table2.column1,....  
2. FROM table1   
3. INNER JOIN table2  
4. ON table1.matching_column = table2.matching_column;  

Query

1. SELECT EMPLOYEE.EMP_NAME, PROJECT.DEPARTMENT   
2. FROM EMPLOYEE  
3. INNER JOIN PROJECT  
4. ON PROJECT.EMP_ID = EMPLOYEE.EMP_ID;  

Output

EMP_NAME	DEPARTMENT
Angelina	Testing
Robert	Development
Christian	Designing
Kristen	Development

2. LEFT JOIN

The SQL left join returns all the values from left table and the matching values from the right table. If there is no matching join value, it will return NULL.

Syntax

1. SELECT table1.column1, table1.column2, table2.column1,....  
2. FROM table1   
3. LEFT JOIN table2  
4. ON table1.matching_column = table2.matching_column;  

Query

1. SELECT EMPLOYEE.EMP_NAME, PROJECT.DEPARTMENT   
2. FROM EMPLOYEE  
3. LEFT JOIN PROJECT  
4. ON PROJECT.EMP_ID = EMPLOYEE.EMP_ID;  

Output

EMP_NAME	DEPARTMENT
Angelina	Testing
Robert	Development
Christian	Designing
Kristen	Development
Russell	NULL
Marry	NULL

3. RIGHT JOIN

In SQL, RIGHT JOIN returns all the values from the values from the rows of right table and the matched values from the left table. If there is no matching in both tables, it will return NULL.

Syntax

1. SELECT table1.column1, table1.column2, table2.column1,....  
2. FROM table1   
3. RIGHT JOIN table2  
4. ON table1.matching_column = table2.matching_column;  

Query

1. SELECT EMPLOYEE.EMP_NAME, PROJECT.DEPARTMENT   
2. FROM EMPLOYEE  
3. RIGHT JOIN PROJECT  
4. ON PROJECT.EMP_ID = EMPLOYEE.EMP_ID;  

Output

EMP_NAME	DEPARTMENT
Angelina	Testing
Robert	Development
Christian	Designing
Kristen	Development

4. FULL JOIN

In SQL, FULL JOIN is the result of a combination of both left and right outer join. Join tables have all the records from both tables. It puts NULL on the place of matches not found.

Syntax

1. SELECT table1.column1, table1.column2, table2.column1,....  
2. FROM table1   
3. FULL JOIN table2  
4. ON table1.matching_column = table2.matching_column;  

Query

1. SELECT EMPLOYEE.EMP_NAME, PROJECT.DEPARTMENT   
2. FROM EMPLOYEE  
3. FULL JOIN PROJECT   
4. ON PROJECT.EMP_ID = EMPLOYEE.EMP_ID;  

Output

EMP_NAME	DEPARTMENT
Angelina	Testing
Robert	Development
Christian	Designing
Kristen	Development
Russell	NULL
Marry	NULL