The σ symbol in relational algebra denotes the select operation which selects tuples based on a given condition. The selection operation is similar to the WHERE clause in SQL queries.
The selection operation has the following general syntax:
σ condition (Relation).
Where:
σ condition represents the selection symbol sigma.
The condition determines which rows are selected.
Relation represents the relation (table) from which rows are taken.
For example, if a relation PASSENGERS has attributes (ID, Name, Age), a selection operation can be performed to get passengers over the age of 18:
PASSENGERS: σAge > 18(PASSENGERS)
This operation selects rows from the PASSENGERS relation where the Age attribute is greater than 18.