Unary Operations

We start the discussion of the relational algebra by examining the two unary operations:
Selection and Projection.
Selection (or Restriction)
spredicate(R) The Selection operation works on a single relation R and defines a
relation that contains only those tuples of R that satisfy the specified
condition ( predicate).
Example 4.1 Selection operation
List all staff with a salary greater than £10,000.
σsalary > 10000(Staff)
Here, the input relation is Staff and the predicate is salary > 10000. The Selection operation
defines a relation containing only those Staff tuples with a salary greater than £10,000. The
result of this operation is shown in Figure 4.2. More complex predicates can be generated
using the logical operators ∧ (AND), ∨ (OR) and ~ (NOT).
Projection
Ða1, . . . , an(R) The Projection operation works on a single relation R and defines a
relation that contains a vertical subset of R, extracting the values of
specified attributes and eliminating duplicates.
Example 4.2 Projection operation
Produce a list of salaries for all staff, showing only the staffNo, fName, lName, and
salary details.
ÐstaffNo, fName, lName, salary(Staff)
In this example, the Projection operation defines a relation that contains only the designated
Staff attributes staffNo, fName, lName, and salary, in the specified order. The result of
this operation is shown in Figure 4.3.