Basic Structure

1. Basic structure of an SQL expression consists of select, from and where clauses.
   • select clause lists attributes to be copied - corresponds to relational algebra project.
   • from clause corresponds to Cartesian product - lists relations to be used.
   • where clause corresponds to selection predicate in relational algebra.
2. Typical query has the form

   

   where each represents an attribute, each a relation, and is a predicate.

3. This is equivalent to the relational algebra expression

   

   • If the where clause is omitted, the predicate P is true.
   • The list of attributes can be replaced with a * to select all.
   • SQL forms the Cartesian product of the relations named, performs a selection using the predicate, then projects the result onto the attributes named.
   • The result of an SQL query is a relation.
   • SQL may internally convert into more efficient expressions.
4. A simple example:

   

   will find the names of all branches in the deposit relation.