Modification of the Database
The content of the database may be modified using the following operations:
All these operations are expressed using the assignment operator ( ←).
1.Deletion
A delete request is expressed similarly to a query, except instead of displaying tuples to the user, the selected tuples are removed from the database.
In deletion, tuples are deleted from the relation.
You can only delete whole tuples; you cannot delete values from specific attributes.
A deletion is expressed in relational algebra by: r ← r - E
Where r is a relation and E is a relational algebra expression.
2.Insertion
Similar to deletion, but uses the union operator (∪) instead of the difference operator (-).
In insertion, tuples are added to the relation.
To insert data into a relation, you can either:
Specify a tuple to be inserted, or
Write a query whose result is a set of tuples to be inserted.
An insertion is expressed in relational algebra by: r ← r ∪ E
Where r is a relation and E is a relational algebra expression.
The insertion of a single tuple is expressed by letting E be a constant relation containing one tuple.
Examples: 1.Insert information into the database specifying that Smith has $1200 in account A-973 at the Perryridge branch.
2.Insert tuples based on the result of a query:
⇒ Provide a $200 savings account as a gift for all loan customers at the Perryridge branch. Let the loan number serve as the account number for the new savings account.
3.Updating
Updating is a mechanism to change a value in a tuple without changing all values in the tuple.
Use the generalized projection operator to perform this task.
Each Fi can be either:
The attribute of r, or
An expression involving only constants and the attributes of r that gives the new value for the attribute.
Note:The schema of the expression resulting from the generalized projection expression must match the original schema of r.
