Difference between a theta join, equijoin and natural join

Question

I\'m having trouble understanding relational algebra when it comes to theta joins, equijoins and natural joins. Could someone please help me better understand it? If I use t

Answer 1

Theta Join: When you make a query for join using any operator,(e.g., =, <, >, >= etc.), then that join query comes under Theta join.

Equi Join: When you make a query for join using equality operator only, then that join query comes under Equi join.

Example:

> SELECT * FROM Emp JOIN Dept ON Emp.DeptID = Dept.DeptID;
> SELECT * FROM Emp INNER JOIN Dept USING(DeptID)

This will show:
 _________________________________________________
| Emp.Name | Emp.DeptID | Dept.Name | Dept.DeptID |
|          |            |           |             |

Note: Equi join is also a theta join!

Natural Join: a type of Equi Join which occurs implicitly by comparing all the same names columns in both tables.

Note: here, the join result has only one column for each pair of same named columns.

Example

 SELECT * FROM Emp NATURAL JOIN Dept

This will show:
 _______________________________
| DeptID | Emp.Name | Dept.Name |
|        |          |           |