SELECT (Relational Algebra)

The SELECT operation, $\sigma$, is used to select a subset of tuples from a relation based on a selection condition. The condition acts as a filter and only keeps those tuples that satisfy the qualifying condition.

$${\sigma }_{<\text{selection condition}>}(R)$$

Where $\sigma$ denotes the SELECT operator. Where $R$ denotes the relation.

Example:

• Select the EMPLOYEE tuples whose department number if 4: ${\sigma }_{DNO=4}(EMPLOYEE)$
• Select the EMPLOYEE tuples whose salary is greater than £30,000: ${\sigma }_{SALARY>30,000}(EMPLOYEE)$

Properties:

• SELECT produces a relation $S$ that has the same schema as $R$.
• SELECT is commutative: ${\sigma }_{<1>}({\sigma }_{<2>(R)})={\sigma }_{<2>}({\sigma }_{<1>(R)})$.
• May be applied in any order.
• We can replace a cascade with a single conjunction. $${\sigma }_{<1>}({\sigma }_{<2>(R)})={\sigma }_{<1>\text{ AND }<2>}(R)$$