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
select A1,A2, ....,An
from r1, r2, ...., rm
where P

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

3. This is equivalent to the relational algebra expression

â…¡A1,A2,....;An(∑P (r1 × r2 ×....×rm))

• 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 effcient  expressions.