Basic query operations in a relational system are selection,
projection, and join. The SELECT statement implements all of them.
Projections and Restrictions
A projection is a subset of the columns in a table. A restriction (also called selection) is a subset of the rows in a table, based on some conditions.
Joins
A join links the rows in two or more tables by comparing the values in key columns and returning rows that have matching values. For example, this query joins the SalesOrderItems, Products, Employees, and SalesOrders tables to calculate sales totals and revenue: