Learning PostgreSQL 11 - Third Edition

The SELECT and PROJECT operations

SELECT is used to restrict tuples from the relation. SELECT always returns a unique set of tuples; that is inherited from entity integrity constraint. For example, the query give me the customer information where the customer_id equals 2 is written as follows:

σ_{customer_id =2} customer

The selection, as mentioned earlier, is commutative; the query give me all customers where the customer's email is known, and the customer's first name is kim is written in three different ways, as follows:

σ_{email is not null}(σ_{first_name =kim} customer)

σ_{first_name =kim}(σ_{email is not null} customer)

σ_{first_name =kim and email is not null} (customer)

The selection predicates are certainly determined by the data types. For numeric ...