- Relational Algebra:
- is a mathematical language for manipulating sets
- Recall that a
Relation is:
- is a set of tuples
Example:
ssn fname lname salary dname --------- ------ -------- --------- --------------- 123456789 John Smith 20900.00 Research 333445555 Frankl Wong 50000.00 Research 999887777 Alicia Zelaya 25000.00 Administration <-- 1 tuple 987654321 Jennif Wallace 43000.00 Administration 666884444 Ramesh Narayan 38000.00 Research 453453453 Joyce English 25000.00 Research 987987987 Ahmad Jabbar 25000.00 Administration 888665555 James Borg 55000.00 Headquarters
- is a set of tuples
- Operations of Relational
Algebra can be divided into
3 categories:
- Set operators:
- ∪ = set union
- ∩ = set intersection
- − = set difference
- × = cartesian product
- Relational database specific operators:
- σ = the selection operator
- π = the projection operator
- ⋈ = the join operator
- Aggregate function:
- sum: find the sum of a set of values
- avg: find the average of a set of values
- count: find the cardinality of a set of values
- any: find the existance of a set of values
- max: find the maximum of a set of values
- min: find the minumum of a set of values
- Set operators:
- Important note:
- A relation
is a set (of tuples)
- Every operation/function in Relational Algebra operates on input sets and returns an output set as result !!!
Graphically:
- A relation
is a set (of tuples)