Object%20Query%20Language

1
Object Query Language

• The need for ad hoc querying
• What SQL does
• What OQL must do
• OQL syntax and examples

2
Ad hoc Querying

• An ad hoc query takes a database and returns some
  data from it
• identifying some parts of the database of
  interest
• and specifying criteria which the data returned
  must satisfy
• It doesnt specify how the query should be
  carried out

3
What SQL Queries Do

• The query takes
• a set of relations (tables or queries)
• a boolean expression on the columns
• a set of columns
• and returns
• a relation (always)
• SQL also provides constructs for data definition
  and data manipulation

4
What OQL Queries Must Do

• Take data from the object model
• and a boolean criterion
• Return data in the object model
• So this is not as simple since there is more
  variety in the data model

5
Features of OQL

• queries are a superset of SQL select queries - no
  update or data definition facilities
• however, a query can invoke an operation on an
  object and this operation can cause an update to
  occur.
• not computationally complete
• OQL is declarative and hence optimisable
• Operators can be freely composed as long as they
  obey the type system

6
More Features of OQL

• A query can return any value expressable in the
  object model
• usually this will be a collection
• but it could be a list, a record, an integer or a
  single object
• It is designed to support two kinds of usage
• interactively - keyboard input with results
  displayed immediately
• embedded in a program - the queries are written
  as part of the program and the results returned
  to the program
• Implementation vary!

7
Simple Queries

• Any expression over values in the data model is
  an OQL query
• an atomic value 5 23 10
• using path expressions superUser . incoming-gtsize
• creating a record struct( name superUser .
  name, unread superUser . incoming . size( )
  )
• creating a collection list( 'A', 'b', 'c' )
• an object superUser
• creating an object UserDetails(userNamesuperUser
  .name, userLogin superUser.login )

8
Collections as Query Results

• Collections can be returned as the result of a
  query in four ways
• by explicit creation - as shown in the previous
  slide
• set( 7, 5, 23 )
• as the result of a select query
• select distinct U from U in theUsers
• as the result of accessing a multi-valued
  property of an object
• superUser . storage
• as the value of a persistent root - very often
  this will be the extent of a type and hence be a
  collection
• theUsers

9
Manipulating Collections

•  Union, intersection, difference of unordered
  collections
• superUser . storage union set( superUser .
  incoming )
•  Quantification operations
• for all U in theUsers U . incoming . size( ) gt
  0
• exists U in theUsers U . incoming . size( ) gt 0
•  select queries iterate over collections
• select U . name from U in theUsers
•  

10
Manipulating Collections II

• Individual elements can be extracted from
  singleton collections
• element( list( 3 ) )
• Individual elements can be extracted from an
  ordered collection by its position
• list( 23.1, 44.2, 39.7, 12.4 ) 2
• "123456789"5
• Portions of ordered collections can also be
  retrieved.
• list( 5, 7, 9, 11, 13 ) 23
• "123456789"57

11
Manipulating Collections III

• Tests for membership of a collection
• 6 in list( 5, 7, 9, 11, 13 )
• '6' in "123456789"
• Aggregation operations
• count( superUser . storage )
• Collections can be turned from one form to
  another.
• listtoset( superUser . incoming . holds )
• distinct( select U . name from U in theUsers )
• flatten( set( set( 1, 2, 3 ), set( 4, 5 ), set(
  6, 7, 8, 9 ) ) )
• this returns set( 1, 2, 3, 4, 5, 6, 7, 8, 9 )

12
Select Queries

• These are the same as SQL
• select U . incoming . size( )
• from U in theUsers
• order by U . name
• This returns a collection of sizes

13
Group By

• This has a slightly different effect
• select U . name from U in theUsers
• group by inBoxSize U . incoming . size
• which returns a collection of pairs of the size
  and the collection of names of people with that
  size
• ( inBoxSize 0, partition bag("Jill", "Joe",
  "Pan") ),
• ( inBoxSize 5, partition bag("John", "Kwame"
  ) ), etc.
• Add in the following to return only those sizes
  with fewer than four users
• having count( select N from partition ) lt 4

14
Named Queries

• A query (view) for how many messages in jbowns
  "TODO box
• define TODOsize as
• select M-gtsize from U in theUsers, M in U .
  storage
• where M . name "TODO" and U . login
  "jbrown"
• which can then be executed by
• TODOsize
• Named queries can be parameterised as in the
  example
• define IncomingSize( loginName ) as
• select U . incoming. size from U in theUsers
• where U . login loginName
• which could be used as follows
• IncomingSize( "jbrown" )

15
Type Coercion or Downcasting

• Sometimes it is necessary to make an object more
  specific
• e.g. given a collection of vehicles some of which
  are members of the car subclass
• to get the number of wheels for a particular car
• select ( (Car) V ). wheels
• from V in theVehicles
• where V . registration "KLS 504D
• V is a vehicle but (Car) V is a car

16
Embedded OQL

• There are three techniques used
• as a free standing function - deprecated in the
  most recent version, but e.g. in O2C
• o2query( aUser, "element( select U from U in
  theUsers where U . login 1 )", userLogin )
• as a Query class
• as methods on the most general collection class

17
The Java Query Class

• This is defined as
• class OQLQuery
• public OQLQuery()
• public OQLQuery(String Query) set query
• public OQLQuery create(String Query) assign
• public void bind(Object param) set param
  value
• public Object execute() execute the query

18
Query Methods on ODMG Collections

• query retrieves the results of an OQL query into
  a collection object.
• the collection variable to be populated and the
  selection condition are passed as collection and
  string parameters
• select_element retrieves the single result of
  the query
• the condition is passed as a string parameter,
  the result is an object of some kind
• exists_element tests if there are any results to
  this query
• the selection condition is passed as a string
  parameter, the result is a boolean
• select returns an iterator over the collection of
  results.
• the query is passed as a string parameter, the
  result is a collection of some kind

19
Query Method on Java Collections

• In Java there is only one method which filters a
  collection.
• It has the following form
• Collection query( String Filter )
• Example
• Bag Unreadusers theUsers.query(
• "this.incoming.length() gt 0" )

20
Example

• User JB theUsers.query( "select U in this
  where login \"jbrown\" ")
• OQLquery query new OQLQuery(
• "select subject from M in MB.holds where
  M.sender 1 and MB in select U.incoming
  from U in theUsers" )
• query.bind( JB )
• Bag result (Bag) query.execute()