Consider the relation PERSON. If you want to display details of persons having age less than or equal to 30 than the select operation will be used as follows:
σ AGE <=30 (PERSON)
The resultant relation will be as shown:
Pocket 2, Mayur
1) Select operation is commutative; i.e.,
σ (σ (R)) = σ (σ (R))
Therefore, series of select can be applied in any order
2) More than one condition can be applied using Boolean operators OR & AND etc.