2 r algebra
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Relational algebra is a set of operations used to specify retrieval requests in the relational model. The basic operations produce new relations by manipulating existing relations. Operations include unary operations like select and project, and binary operations like union, intersection, difference, cartesian product, and join. A join operation identifies and selects related tuples from two relations based on a join condition.
2 r algebra
- 1. 2. Relational Algebra Definition: The basic set of operations for the relational model. And a user enable to specify basic retrieval requests. ☞ The algebra operations produce new relations, which can be further manipulated using operations of the same algebra. ☞ A sequence forms a relational algebra expression ☞ Has a procedural paradigm: you need to tell what/how to construct the result
- 2. 2. Relational Algebra Basic Relational Operation: ◆ Unary operations Select: σ. Project: or π. The rename operator: ρ. ◆ Binary operations Set operations: Union: Intersection: Difference: – Cartesian product Join operations:
- 3. 2. Relational Algebra ◆ Unary operations Select Operation: Used to select a subset of the tuples from a relation that satisfy a selection condition. It is a filter that keeps only those tuples that satisfy a qualifying condition. Example To select the employee tuples whose department number is four or those whose salary is greater than $30,000 the following notation is used: Σdno = 4 (employee) Σsalary > 30,000 (employee) In general, the select operation is denoted by σ <selection condition>(r) where the symbol σ (sigma) is used to denote the select operator, and the selection condition is a boolean expression specified on the attributes of relation r.
- 4. 2. Relational Algebra Rename Operation: To apply several relational algebra operations one after the other. Relational algebra expression by nesting the operations, or apply one operation at a time and create intermediate result relations. In the latter case, we must give names to the relations that hold the intermediate results. Example To retrieve the first name, last name, and salary of all employees who work in department number 5, we must apply a select and a project operation. We can write a single relational algebra expression as follows: Πfname, lname, salary(σ dno=5(employee)) Or explicitly show the sequence of operations, giving a name to each Intermediate relation: Dep5_emps ← σ dno=5(employee) Result ← π fname, lname, salary (dep5_emps)
- 5. 2. Relational Algebra ◆ Binary Relational Operations ▪ Relational Algebra Operations from Set theory Union Operation: Denoted by r s, is a relation that includes all tuples that are either in r or in s or in both r and s. Example To retrieve the social security numbers of all employees who either work in department 5 or directly supervise an employee who works in department 5, we can use the union operation as follows: Dep5_emps ← σdno=5 (employee) Result1 ← π ssn(dep5_emps) Result2(ssn) ← π superssn(dep5_emps) Result ← result1 result2 The union operation produces the tuples that are in either result1 or result2 or both. The two operands must be “type compatible”.
- 7. 2. Relational Algebra Join Operation: • The sequence of Cartesian product followed by select is used quite commonly to identify and select related tuples from two relations, a special operation, called join. It is denoted by a . • This operation is very important for any relational database with more than a single relation, because it allows us to process relationships among relations. • The general form of a join operation on two relations r(a1, a2, . . ., an) and s(b1, b2, . . ., bm) is: R <join condition>s Where r and s can be any relations that result from general relational algebra expressions.
- 8. 2. Relational Algebra Join Operation: Example: Suppose that we want to retrieve the name of the manager of each department. To get the manager’s name, we need to combine each department tuple with the employee tuple whose ssn value matches the mgrssn value in the department tuple. We do this by using the join Dept_mgr ← department operation. mgrssn=ssn(mployee).