Slide 16.11: Access to sets of objects

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Slide 16.10: Equal treatment of attributes and methods
Slide 16.12: Implications of a class hierarchy
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Desired Properties (Cont.)

Access to Sets of Objects (Cont.)

It is possible to name sets of objects.

myfamily := SELECT p FROM p IN person
  WHERE name = myname;
SELECT name FROM myfamily WHERE age > 50;

It should be possible to use in the from-clause any kind of collection that can be derived from the database. Searches for the names of manufacturers of blue vehicles.
SELECT name FROM ( SELECT manufacturer FROM vehicle WHERE color = 'blue' );

The use of collections in queries can be extended to where-clause. Determines the number of companies whose presidents‘. annual salary exceeds 200,000 dollars.
SELECT count(f) FROM f IN company WHERE president IN ( SELECT a FROM a IN employee WHERE salary > 200000 );

Set-valued attributes or methods in path expressions are problematic. Determines the salaries of employees working in companies‘. subsidiaries.
SELECT f.subsidiaries.employees.salary FROM f IN company;
To avoid a type error, we cannot apply attribute employees to the result of f.subsidiaries, because the latter is a set, employees is defined only for single subsidiaries. Two solutions are
1. Sets whose elements are themselves sets are by default unnested.
2. Unnest sets explicitly by means of a flatten operator.
  flatten( SELECT flatten( flatten(f.subsidiaries).emplyees).salary FROM f IN company);

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Slide 16.10: Equal treatment of attributes and methods
Slide 16.12: Implications of a class hierarchy
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Waiter: Can I take your order sir, kids eat free today.
Me: Oh, well in that case I’ll just have a glass of water and
my son will have the grilled lobster,
a 15oz steak and a small bottle of champagne please.