- Different operations that modify the contents of the database are
- Delete
- Insert
- Update
- We express database modification by using the assignment operation.
Deletion
- The deletion operation removes the selected tuples from the database using delete values of any particular attribute.
- In relational algebra a deletion is expressed by
- r ← r - E
- r is a relation
- E is a relational algebra query
Example
Delete 'Ramesh' Record from the database
- employees ← employees - σemp_name='Ramesh'(employees)
- Delete all employee working in IT department
- employees ← employees - σdepartment='IT'(employees)
Insertion
- The relational algebra expresses an insertion by
- r ← r Ս E
- r is a relation
- E is relational algebra expression
Examples
- Insert Ramesh Record in employee relation
- employees ← employees Ս {('E004','Ramesh', 12000)}
Updating
- Sometimes, we wish to change a value in a tuple without changing all values in the tuple. We can use generalized projection operation to do this:
- r ← ∏f1,f2,f3...fn(r)
- Where fi the ith attribute to be updated has expression involving constants and attributes of r, that gives new value for the attribute.
- Example
- ∏emp_id, salary*1.05(employees)