CS145 Lecture Notes (13) -- Object-Relational SQL

Most major DBMS vendors call their products "object-relational."
There is great variation in OO functionality among current products.
Rumor is that OO features are used much less than you might expect, especially considering the effort put into them.
We'll cover a portion of the SQL-99 standard, as in the textbook.
Oracle has object support that does not adhere exactly to the standard. (See our Oracle help document if interested.)

SQL-99 Object Support - Major Components

Type definitions separate from table definitions
User-defined simple and complex types
Methods
References (pointers)
Observer, generator, and mutator functions
Equality and ordering relationships

Type Definitions (UDTs)

Separate from table definitions
Can use for multiple tables
Constraints on per-table basis
Can use for attribute types (nested structure)

Original student table, recast:

  CREATE TYPE StudentType
      (ID integer, name char(30), address char(100), GPA float, sizeHS integer)

  CREATE TABLE Student OF StudentType (PRIMARY KEY (ID))

Effect on queries:

Attribute references implicitly invoke "observer methods"
FROM-clause table references require variables

Example: Find name, address of all students with GPA > 3.8 and sizeHS > 1000

Can use same type for multiple relations:

  CREATE TABLE OldStudent OF StudentType (PRIMARY KEY (name,address))

Note: Constraints are part of table declaration, not type declaration

Nested Structures

An attribute of a table can have a User-Defined Type (UDT)

Example: Use StudentType in student phone list:

   CRATE TABLE Phone (student StudentType, number char(15))

But might better be done with references (see below)

Example: More structure in Student definition:

  CREATE TYPE AddressType AS (street char(50), city char(50), zip integer)

  CREATE TYPE InfoType AS (GPA float, sizeHS integer)

  CREATE TYPE StudentType AS
     (ID integer, name char(30), address AddressType, info InfoType)

  CREATE TABLE Student OF StudentType (PRIMARY KEY (ID))

(picture)

Note: no actual relations with types AddressType or InfoType

Queries with Nesting

Just more dots

Example: Find name, street of all students from Palo Alto with GPA < 3.5

Distinct Types

User-defined type name
Attached to atomic type
Enables stricter type-checking

  CREATE TYPE SatScore AS integer
  CREATE TYPE NumStudents as integer
  CREATE TABLE Student (name char(30), sat SatScore, sizeHS NumStudents)

Now can't inadvertently compare student SAT scores and a High School sizes.

Methods

Signature declared as part of type
Method body implemented using, e.g., PL/SQL, PSM, or Java

Examples:

  CREATE TYPE InfoType AS (GPA float, sizeHS integer)
  METHOD adjustGPA() RETURNS float

  CREATE TYPE AddressType AS (street char(50), city char(50), zip integer)
  METHOD sendto(name char(20), msg char(50)) RETURNS char(1)

Queries with Methods

Example: Return ID and adjusted GPA for all students

Example: Send "sorry" mailing to all students with adjusted GPA < 3.0. Return ID and confirmation character as result of query.

References

To be referenced, tuples must have an explicitly declared "reference column"
Reference column can be derived from primary key or purely system-generated

Example: To make students referenceable:

  CREATE TABLE Student OF StudentType
     (PRIMARY KEY (ID),
      REF IS SRef DERIVED) // alternative is SYSTEM GENERATED

Example: Student phone list:

  CREATE TABLE Phone (student REF(StudentType), number char(15))

Phone tuples can reference any tuples of StudentType, e.g., in Student table and/or in OldStudent table (if it contains "REF IS" clause).

Example: Student phone list without old students:

  CREATE TABLE Phone
   (student REF(StudentType) SCOPE Student, number char(15))

When SCOPE is used, can declare referential-integrity-like constraint enforcement, e.g.:

  REFERENCES ARE CHECKED ON DELETE SET NULL

Queries with References

Dereferencing operator DEREF
Dereferencing and attribute selection operator "->"

Example: Phone numbers of students living in Mountain View

Example: Students with phone numbers containing "(408)"

Main effect of references is to eliminate joins.

Generator and Mutator Functions

For creating and updating values of UDTs
See book

Ordering Relationships

SQL uses equality and ordering of values in lots of places:

  =, <, >, <=, >=, <>
  ORDER BY
  DISTINCT, GROUP BY

Would be nice to have these capabilities for UDTs also

Example: Students ordered by adjusted GPA

Example: Number of students at each address

Capabilities vary from product to product. No product implements the standard ("CREATE ORDERING"). Main concepts:

Strict object equality:
- Two UDT objects are equal if and only if they are the same object.
- Permits operations =, <>, DISTINCT, GROUP BY
Structural equality:
- Two UDT objects are equal if and only if all components are equal (recursively).
- Base case is equality of atomic values
- Permits operations =, <>, DISTINCT, GROUP BY
Method-defined equality:
- Defined method takes two UDT objects and returns = or <>
- Permits operations =, <>, DISTINCT, GROUP BY
Method-defined comparison:
- Defined method takes two UDT objects and returns =, <, or >
- Permits operations =, <, > <=, >=, <>, ORDER BY, DISTINCT, GROUP BY
Method-defined mapping:
- Defined method takes UDT object and returns real value
- Compare two objects by comparing their returned values
- Permits operations =, <, > <=, >=, <>, ORDER BY, DISTINCT, GROUP BY