The Qt IVI Query Language

Automotive systems are getting bigger and bigger and with it also the feature-set in areas like entertainment or connectivity. Modern system can handle phone calls, access the mobile phone's addressbook, and have a mediaplayer which can manage a media database similar to desktop tools like iTunes. As addressbooks and media databases are quite big these days, it's important to be able to filter, sort and search them in a convenient way.

Because of this the Qt IVI Query Language was created and it provides you with an own minimal language to express what exactly you want to be displayed and how it should be sorted.

It was chosen to create a new language to be independent on the underlying architecture and its (potentially) underlying database languages like SQL or PSQL. Other than the two named, the Qt IVI Query Language is not specifying the actual data itself and is just used for filtering and sorting.

Working with the Query Language

Currently the QIviSearchAndBrowseModel is the only class which supports the usage of the query language. In this case the backend of the model informs the query parser about the identifiers available for filtering and sorting.

An identifier is the name of a column or the property which can be used in the query language. Let's say you use the QIviSearchAndBrowseModel to access a list of QIviAudioTrackItems, then the available identfiers will be every property of the QIviAudioTrackItem. Writing the following query would result in a search for tracks on the album "Nevermind":

album='Nevermind'

Filtering and Sorting

The Qt IVI Query Language has two parts: the first part being the filter specification, and the second part defining the sort order.

album='Nevermind' [/trackNumber]

In the query shown above the first part is album='Nevermind', and it indicates to only show tracks which are from the album "Nevermind". The second part is specified by [] and defines the ordering: in this case the result should be sorted by trackNumber in ascending order.

Filtering

For filtering a result the following operators can be used to match a identifier to a value:

OperatorDescription
=Tests whether the identifier's value is equal to the passed value. In case of a string the comparison is case-sensitive. The == operator is an alias and delivers the same result.
!=Tests whether the identifier's value is not equal to the passed value. In case of a string the comparison is case-sensitive.
~=Tests whether the identifier's value is equal to the passed value, but the comparison is done case-insensitive.
>Tests whether the lefthand-side value is greater than the righthand-side value. (only works for numbers).
>=Tests whether the lefthand-side value is greater than or equal to the righthand-side value. (only works for numbers).
<Tests whether the lefthand-side value is less than the righthand-side value. (only works for numbers).
<=Tests whether the lefthand-side value is less than or equal to the righthand-side value. (only works for numbers).

When comparing an identifier against a string, the string always needs to be encapsulated either between two quotation marks - either single ' or double " ones. Numbers are supported in various formats, including signed and unsigned numbers and floats using an exponent e.g. -5.0E15.

Sorting

The second part of a query is used for sorting the result. It's optional and if not provided, the backend can decide in which order the result should be listed. To sort by trackNumber in ascending order use the following code:

[/trackNumber]

to sort in descending order:

[\trackNumber]

Using this query on a big list of songs might not give the intendend result as there could be two different tracks having the same trackNumber. For this case multiple sort orders can be provided.

[\trackNumber][/album]

The above query would sort the tracks by trackNumber in descending order and the track with the same trackNumber by album name in ascending order.

Conjunctions

Only adding a specific condition and defining the order is often not enough to achieve the wanted result. The Qt IVI Query Language also supports basic boolean algebra. Multiple filters can be combined by using AND / OR conjunctions. The following query would only list the first 5 tracks of the album 'Metallica':

album='Metallica' & trackNumber<=5

Of course it's also supported to use brackets as well as to negate filters:

(album='Metallica') & !(trackNumber>5)

Backend integration

For the backend, the query is translated from a string to a binary representation similar to an AST. Like the query itself, the representation is split into two parts: the first part is a pointer to a QIviAbstractQueryTerm which can be cast to one of the following types:

QIviConjunctionTerm

The representation of a conjunction between two query terms

QIviFilterTerm

The representation of a filter

QIviScopeTerm

The representation of a scope which can hold another term

The second part is a QList<QIviOrderTerm> and provides the ordering the user wants the result to be in.

Usually this C++ representation is translated into whatever form the backend supports. For getting the results from an SQL database this can look like this:

First we need a function to translate the identifiers used in the query to the column names used in the database:

QString SearchAndBrowseBackend::mapIdentifiers(const QString &type)
{
    if (type == QLatin1String("artist"))
        return QLatin1String("artistName");
    else if (type == QLatin1String("album"))
        return QLatin1String("albumName");
    else if (type == QLatin1String("track"))
        return QLatin1String("trackName");
    else
        return type;
}

Next we need functions to translate the QueryTerms and the OrderTerms into SQL clauses:

QString SearchAndBrowseBackend::createSortOrder(const QString &type, const QList<QIviOrderTerm> &orderTerms)
{
    QStringList order;
    int i = 0;
    for (const QIviOrderTerm & term : orderTerms) {
        if (i)
            order.append(",");

        order.append(mapIdentifiers(term.propertyName()));
        if (term.isAscending())
            order.append("ASC");
        else
            order.append("DESC");

        i++;
    }

    return order.join(' ');
}

QString SearchAndBrowseBackend::createWhereClause(QIviAbstractQueryTerm *term)
{
    if (!term)
        return QString();

    switch (term->type()) {
    case QIviAbstractQueryTerm::ScopeTerm: {
        QIviScopeTerm *scope = static_cast<QIviScopeTerm*>(term);
        return QString(QLatin1String("%1 (%2)")).arg(scope->isNegated() ? "NOT" : "",createWhereClause(scope->term()));
    }
    case QIviAbstractQueryTerm::ConjunctionTerm: {
        QIviConjunctionTerm *conjunctionTerm = static_cast<QIviConjunctionTerm*>(term);
        QString conjunction = QLatin1String("AND");
        if (conjunctionTerm->conjunction() == QIviConjunctionTerm::Or)
            conjunction = QLatin1String("OR");

        QString string;
        QListIterator<QIviAbstractQueryTerm*> it(conjunctionTerm->terms());
        while (it.hasNext()) {
            string += createWhereClause(it.next());
            if (it.hasNext())
                string += QLatin1Literal(" ") + conjunction + QLatin1Literal(" ");
        }
        return string;
    }
    case QIviAbstractQueryTerm::FilterTerm: {
        QIviFilterTerm *filter = static_cast<QIviFilterTerm*>(term);
        QString operatorString;
        bool negated = filter->isNegated();
        QString value;
        if (filter->value().type() == QVariant::String)
            value = QString(QLatin1String("'%1'")).arg(filter->value().toString().replace('*', '%'));
        else
            value = filter->value().toString();

        switch (filter->operatorType()){
            case QIviFilterTerm::Equals: operatorString = QLatin1String("="); break;
            case QIviFilterTerm::EqualsCaseInsensitive: operatorString = QLatin1String("LIKE"); break;
            case QIviFilterTerm::Unequals: operatorString = QLatin1String("="); negated = !negated; break;
            case QIviFilterTerm::GreaterThan: operatorString = QLatin1String(">"); break;
            case QIviFilterTerm::GreaterEquals: operatorString = QLatin1String(">="); break;
            case QIviFilterTerm::LowerThan: operatorString = QLatin1String("<"); break;
            case QIviFilterTerm::LowerEquals: operatorString = QLatin1String("<="); break;
        }

        QStringList clause;
        if (negated)
            clause.append(QLatin1String("NOT"));
        clause.append(mapIdentifiers(filter->propertyName()));
        clause.append(operatorString);
        clause.append(value);

        return clause.join(" ");
    }
    }

    return QString();
}

The actual query can then be created like this:

QString query = QString(QLatin1String("SELECT * FROM tracks WHERE %1 ORDER BY %2")).arg(createWhereClause(queryTerm), createSortOrder(orderTerms));

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