5. SQL queries

A SQL query corresponds to a series of operations on the database. The query engine receives a request from the application. This query is declarative, i.e. it doesn't say what operations to perform, or in what order. It is first transformed into an imperative program, which is then executed to calculate the result of the query. The program corresponding to the query is called an execution plan. Several steps are required to obtain an execution plan.

First, the syntax of the SQL query is checked. The query must conform to the grammar of the SQL language. The relationship names in the FROM clause must correspond to a database relationship. Attribute names in other clauses (SELECT, WHERE, ORDER BY and GROUP BY) must correspond unambiguously to an attribute in a relationship. Type matching is checked for each predicate, e.g. price < 9.5 is not correct if the price...