Wt  4.0.2
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Wt::Dbo::Query< Result, BindStrategy > Class Template Reference

A database query. More...

#include <Wt/Dbo/Query.h>

Public Member Functions

 Query ()
 Default constructor.
 
 ~Query ()
 Destructor.
 
 Query (const Query &other)
 Copy constructor.
 
Queryoperator= (const Query &other)
 Assignment operator.
 
std::vector< FieldInfofields () const
 Returns the result fields.
 
Sessionsession () const
 Returns the session.
 
template<typename T >
Query< Result, BindStrategy > & bind (const T &value)
 Binds a value to the next positional marker. More...
 
void reset ()
 Resets bound values. More...
 
Result resultValue () const
 Returns a unique result value. More...
 
collection< Result > resultList () const
 Returns a result list. More...
 
 operator Result () const
 Returns a unique result value. More...
 
 operator collection< Result > () const
 Returns a result list. More...
 
Methods for composing a query (DynamicBinding only)
Query< Result, BindStrategy > & where (const std::string &condition)
 Adds a query condition. More...
 
Query< Result, BindStrategy > & orderBy (const std::string &fieldName)
 Sets the result order. More...
 
Query< Result, BindStrategy > & groupBy (const std::string &fields)
 Sets the grouping field(s). More...
 
Query< Result, BindStrategy > & having (const std::string &fields)
 Sets the grouping filter(s). More...
 
Query< Result, BindStrategy > & offset (int count)
 Sets a result offset. More...
 
int offset () const
 Returns an offset set for this query. More...
 
Query< Result, BindStrategy > & limit (int count)
 Sets a result limit. More...
 
int limit () const
 Returns a limit set for this query. More...
 

Detailed Description

template<class Result, typename BindStrategy = DynamicBinding>
class Wt::Dbo::Query< Result, BindStrategy >

A database query.

The query fetches results of type Result from the database. This can be any type for which query_result_traits are properly implemented. The library provides these implementations for primitive values (see sql_value_traits), database objects (ptr) and boost::tuple.

Simple queries can be done using Session::find(), while more elaborate queries (with arbitrary result types) using Session::query().

You may insert positional holders anywhere in the query for parameters using '?', and bind these to actual values using bind().

The query result may be fetched using resultValue() or resultList().

Usage example:

typedef Wt::Dbo::ptr<Account> AccountPtr;
Wt::Dbo::Query<AccountPtr> query = session.find<Account>().where("balance > ?").bind(100000);
Accounts accounts = query.resultList();
for (Accounts::const_iterator i = accounts.begin(); i != accounts.end(); ++i)
std::cerr << "Name: " << (*i)->name << std::end;

The BindStrategy specifies how you want to bind parameters to your query (if any).

When using DynamicBinding (which is the default), parameter binding to an actual sql statement is deferred until the query is run. This has the advantage that you can compose the query definition using helper methods provided in the query object (where(), groupBy(), having() and orderBy()), possibly intermixing this with parameter binding, and you can keep the query around and run the query multiple times, perhaps with different parameter values or to scroll through the query results. The where(), groupBy(), having(), and orderBy() are merely convenience methods which you may use to compose the querys incrementally, but you may just as well specify the entire SQL as a single string.

When using DirectBinding, parameters are directly bound to an underlying sql statement. Therefore, the query must be specified entirely when created. Because of this reliance on an sql statement, it can be run only once (one call to resultValue() or resultList()) after which it should be discarded. You should not try to keep a query object around when using this parameter binding strategy (that will amost always not do what you would hope it to do).

Member Function Documentation

template<class Result, typename BindStrategy = DynamicBinding>
template<typename T >
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::bind ( const T &  value)

Binds a value to the next positional marker.

This binds the value to the next positional marker in the query condition.

template<class Result, typename BindStrategy = DynamicBinding>
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::groupBy ( const std::string &  fields)

Sets the grouping field(s).

This is a convenience method for creating a SQL query, and sets a group by field expression for the current query.

Groups results based on unique values of the indicated field(s), which is a comma separated list of fields. Only fields on which you group and aggregate functions can be selected by a query.

A field that refers to a database object that is selected by the query is expanded to all the corresponding fields of that database object (as in the select statement).

Note
This method is not available when using a DirectBinding binding strategy.
template<class Result, typename BindStrategy = DynamicBinding>
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::having ( const std::string &  fields)

Sets the grouping filter(s).

It's like where(), but for aggregate fields.

For example you can't go:

select department.name, count(employees) from department where count(employees) > 5 group by count(employees);

Because you can't have aggregate fields in a where clause, but you can go:

select department.name, count(employees) from department group by count(employees) having count(employees) > 5;

This will of course return all the departments with more than 5 employees (and their employee count).

Note
This method is not available when using a DirectBinding binding strategy.
You must have a group by clause, in order to have a 'having' clause
template<class Result, typename BindStrategy = DynamicBinding>
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::limit ( int  count)

Sets a result limit.

Sets a result limit. This has the effect that the next resultList() call will return up to count results. Use -1 to indicate no limit.

This provides the (non standard) limit part of an SQL query.

See also
offset()
Note
This method is not available when using a DirectBinding binding strategy.
template<class Result, typename BindStrategy = DynamicBinding>
int Wt::Dbo::Query< Result, BindStrategy >::limit ( ) const

Returns a limit set for this query.

See also
limit(int)
template<class Result, typename BindStrategy = DynamicBinding>
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::offset ( int  count)

Sets a result offset.

Sets a result offset. This has the effect that the next resultList() call will skip as many results as the offset indicates. Use -1 to indicate no offset.

This provides the (non standard) offset part of an SQL query.

See also
limit()
Note
This method is not available when using a DirectBinding binding strategy.
template<class Result, typename BindStrategy = DynamicBinding>
int Wt::Dbo::Query< Result, BindStrategy >::offset ( ) const

Returns an offset set for this query.

See also
offset(int)
template<class Result, typename BindStrategy = DynamicBinding>
Wt::Dbo::Query< Result, BindStrategy >::operator collection< Result > ( ) const

Returns a result list.

This is a convenience conversion operator that calls resultList().

template<class Result, typename BindStrategy = DynamicBinding>
Wt::Dbo::Query< Result, BindStrategy >::operator Result ( ) const

Returns a unique result value.

This is a convenience conversion operator that calls resultValue().

template<class Result, typename BindStrategy = DynamicBinding>
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::orderBy ( const std::string &  fieldName)

Sets the result order.

This is a convenience method for creating a SQL query, and sets an order by field expression for the current query.

Orders the results based on the given field name (or multiple names, comma-separated).

Note
This method is not available when using a DirectBinding binding strategy.
template<class Result, typename BindStrategy = DynamicBinding>
void Wt::Dbo::Query< Result, BindStrategy >::reset ( )

Resets bound values.

This undoes all previous calls to bind().

template<class Result, typename BindStrategy = DynamicBinding>
collection< Result > Wt::Dbo::Query< Result, BindStrategy >::resultList ( ) const

Returns a result list.

This returns a collection which is backed by the underlying query. The query is not actually run until this collection is traversed or its size is asked.

When using a DynamicBinding bind strategy, after a result has been fetched, the query can no longer be used.

template<class Result, typename BindStrategy = DynamicBinding>
Result Wt::Dbo::Query< Result, BindStrategy >::resultValue ( ) const

Returns a unique result value.

You can use this method if you are expecting the query to return at most one result. If the query returns more than one result a NoUniqueResultException is thrown.

When using a DynamicBinding bind strategy, after a result has been fetched, the query can no longer be used.

template<class Result, typename BindStrategy = DynamicBinding>
Query<Result, BindStrategy>& Wt::Dbo::Query< Result, BindStrategy >::where ( const std::string &  condition)

Adds a query condition.

This is a convenience method for creating a SQL query, and concatenates a new where condition expression to the current query.

The condition must be a valid SQL condition expression.

Multiple conditions may be provided by successive calls to where(), which must each be fulfilled, and are concatenated together using 'and'.

As with any part of the SQL query, a condition may contain positional markers '?' to which values may be bound using bind().

Note
This method is not available when using a DirectBinding binding strategy.

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