Path Expressions

Path expressions are important constructs in the syntax of the query language, for several reasons. First, they define navigation paths through the relationships in the abstract schema. These path definitions affect both the scope and the results of a query. Second, they can appear in any of the main clauses of a query (SELECT, DELETE, HAVING, UPDATE, WHERE, FROM, GROUP BY, ORDER BY). Finally, although much of the query language is a subset of SQL, path expressions are extensions not found in SQL.

Examples of Path Expressions

Here, the WHERE clause contains a single_valued_path_expression. The p is an identification variable, and salary is a persistent field of Player.

SELECT DISTINCT p
FROM Player p
 WHERE p.salary BETWEEN :lowerSalary AND :higherSalary

Here, the WHERE clause also contains a single_valued_path_expression. The t is an identification variable, league is a single-valued relationship field, and sport is a persistent field of league.

SELECT DISTINCT p
FROM Player p, IN (p.teams) t
 WHERE t.league.sport = :sport

Here, the WHERE clause contains a collection_valued_path_expression. The p is an identification variable, and teams designates a collection-valued relationship field.

SELECT DISTINCT p
FROM Player p
 WHERE p.teams IS EMPTY

Expression Types

The type of a path expression is the type of the object represented by the ending element, which can be one of the following:

• Persistent field

• Single-valued relationship field

• Collection-valued relationship field

For example, the type of the expression p.salary is double because the terminating persistent field (salary) is a double.

In the expression p.teams, the terminating element is a collection-valued relationship field (teams). This expression’s type is a collection of the abstract schema type named Team. Because Team is the abstract schema name for the Team entity, this type maps to the entity. For more information on the type mapping of abstract schemas, see the section Return Types.

Navigation

A path expression enables the query to navigate to related entities. The terminating elements of an expression determine whether navigation is allowed. If an expression contains a single-valued relationship field, the navigation can continue to an object that is related to the field. However, an expression cannot navigate beyond a persistent field or a collection-valued relationship field. For example, the expression p.teams.league.sport is illegal, because teams is a collection-valued relationship field. To reach the sport field, the FROM clause could define an identification variable named t for the teams field:

FROM Player AS p, IN (p.teams) t
 WHERE t.league.sport = ’soccer’