2. Natural Join
• Natural Join combines the tables based on
columns with the same name and data type.
• Automatically matches common columns
between tables.
• SQL: SELECT * FROM Employees NATURAL JOIN
Departments;
• PQL: Employees Departments
⋈
3. emp_id name dept_id
101 Alice 1
102 Bob 2
103 Charlie 1
104 David 3
105 Eva NULL
dept_id dept_name
1 HR
2 Engineering
3 Marketing
4 Finance
Example - Natural Join
emp_id name dept_id dept_name
101 Alice 1 HR
102 Bob 2 Engineering
103 Charlie 1 HR
104 David 3 Marketing
Employees Departments
⋈
Employees Departments
4. Theta Join
• Theta Join combines the tables based on a
specified condition (not necessarily equality).
• Conditional Operators incudes: >,<, <=, >= etc.
SQL –
SELECT * FROM Employees E, Departments D
WHERE E.dept_id < D.dept_id;
PQL-
Employees⋈(E.dept_id<D.dept_id) Departments
5. emp_id name dept_id
101 Alice 1
102 Bob 2
103 Charlie 1
105 Eva NULL
dept_id dept_name
1 HR
2 Engineering
3 Marketing
4 Finance
Example - Theta Join
Employees (E.dept_id < D.dept_id) Departments
⋈
Employees Departments
emp_id name dept_id dept_id dept_name
101 Alice 1 2 Engineering
101 Alice 1 3 Marketing
101 Alice 1 4 Finance
102 Bob 2 3 Marketing
102 Bob 2 4 Finance
103 Charlie 1 2 Engineering
103 Charlie 1 3 Marketing
103 Charlie 1 4 Finance
6. Equi Join
• Joins tables based on equality between
specified columns.
SQL-
SELECT * FROM Employees E, Departments D
WHERE E.dept_id =
D.dept_id;
PQL-
Employees⋈(E.dept_id=D.dept_id) Departments
7. dept_id dept_name
1 HR
2 Engineering
3 Marketing
4 Finance
Example - Equi Join
Employees (E.dept_id = D.dept_id) Departments
⋈
Employees Departments
emp_id name dept_id dept_id dept_name
101 Alice 1 1 HR
102 Bob 2 2 Engineering
103 Charlie 1 1 HR
104 David 3 3 Marketing
emp_id name dept_id
101 Alice 1
102 Bob 2
103 Charlie 1
104 David 3
105 Eva NULL
8. Outer Joins
• Outer Joins Returns matching rows plus
unmatched rows from one or both tables.
• Types:
1. Left Outer Join
2. Right Outer Join
3. Full Outer Join
9. Left Outer Join
• It returns all rows from the left table and
matching rows from the right table.
SQL-
SELECT * FROM Employees E LEFT OUTER JOIN
Departments D ON E.dept_id = D.dept_id;
PQL-
π*
(Employees (E.dept_id=D.dept_id)Departments)
⟕
10. dept_id dept_name
1 HR
2 Engineering
3 Marketing
4 Finance
Example – Left Outer Join
Employees (E.dept_id = D.dept_id) Departments
⟕
Employees Departments
emp_id name dept_id
101 Alice 1
102 Bob 2
103 Charlie 1
104 David 3
105 Eva NULL
emp_id name dept_id dept_id dept_name
101 Alice 1 1 HR
102 Bob 2 2 Engineering
103 Charlie 1 1 HR
104 David 3 3 Marketing
105 Eva NULL NULL NULL
11. Right Outer Join
• It returns all rows from the right table and
matching rows from the left table.
SQL-
SELECT * FROM Employees E RIGHT OUTER JOIN
Departments D ON E.dept_id = D.dept_id;
PQL-
π*
(Employees ⟖ (E.dept_id=D.dept_id)
Departments)
12. dept_id dept_name
1 HR
2 Engineering
3 Marketing
4 Finance
Example – Right Outer Join
Employees (E.dept_id = D.dept_id) Departments
⟖
Employees Departments
emp_id name dept_id
101 Alice 1
102 Bob 2
103 Charlie 1
104 David 3
105 Eva NULL
emp_id name dept_id dept_id dept_name
101 Alice 1 1 HR
103 Charlie 1 1 HR
102 Bob 2 2 Engineering
104 David 3 3 Marketing
NULL NULL NULL 4 Finance
13. Full Outer Join
• It returns all rows when there's a match in
either the left or right table.
SQL –
SELECT * FROM Employees E FULL OUTER JOIN
Departments D ON E.dept_id = D.dept_id;
PQL –
π*(Employees (
⟗ E.dept_id=D.dept_id)
Departments)
14. dept_id dept_name
1 HR
2 Engineering
3 Marketing
4 Finance
Example – Full Outer Join
Employees (E.dept_id = D.dept_id) Departments
⟗
Employees Departments
emp_id name dept_id
101 Alice 1
102 Bob 2
103 Charlie 1
104 David 3
105 Eva NULL
emp_id name dept_id dept_name
101 Alice 1 HR
102 Bob 2 Engineering
103 Charlie 1 HR
104 David 3 Marketing
105 Eva NULL NULL
NULL NULL 4 Finance
15. Join Type Definition
Includes
Matching
Rows?
Includes Non-
Matching Rows?
Natural Join
Joins based on common column(s) with the
same name and data type.
Yes No
Theta Join
Joins based on a condition other than
equality (e.g., <, >, !=).
Yes No
Equi Join
Joins based on equality between specific
columns.
Yes No
Left Outer
Join
Returns all rows from the left table, plus
matching rows from the right table. Non-
matching right table values are filled with
NULL.
Yes Left table only
Right Outer
Join
Returns all rows from the right table, plus
matching rows from the left table. Non-
matching left table values are filled with
NULL.
Yes Right table only
Full Outer
Join
Returns all rows when there is a match in
either table. Non-matching values from both
tables are filled with NULL.
Yes Both tables
