We can then apply a mathematical theory called relational algebra to carry out operations on these tables. Two very common operations are selection and projection . The selection operation specifies a subset of rows in a tables by using a boolean expression involving attributes. OR or Not conditions, which can be evaluated as being either true or false. Thus, selection provides a way to identify those rows in a relation that satisfy a particular condition, such as having regular flowers, simple leaf type, etc.
Projection specifies a subset of columns in a table by listing the attributes.
The projection operation also eliminates duplicates from the resulting rows. It operates on two tables having one or more common attributes. The join operation works well with other operations to produce query data. In relational databases, the same information is often retrievable in multiple ways, and some may be more efficient than others.