Unless clause
You might want to apply an action if certain instances of data or events do not exist. In these cases, you can use the unless clause. It has the same syntax as the using clause, but the logic is essentially reversed; that is, if the queries inside the unless clause are successful, the selection fails and the action is not applied.
The syntax of the unless clause is as follows:
<EventClassName> (<Variable>)
where [<Expression> <CondOperator> <Expression>, ...]
unless {<DataClassName> (<Variable>)
where [<Expression> <CondOperator> <Expression>,
<Expression> <CondOperator> <Expression>,
...];
<DataClassName> (<Variable>)
where [<Expression> <CondOperator> <Expression>,
<Expression> <CondOperator> <Expression>,
...];
... ;
}
where [<Expression> <CondOperator> <Expression>, ...]
unless {<DataClassName> (<Variable>)
where [<Expression> <CondOperator> <Expression>,
<Expression> <CondOperator> <Expression>,
...];
<DataClassName> (<Variable>)
where [<Expression> <CondOperator> <Expression>,
<Expression> <CondOperator> <Expression>,
...];
... ;
}
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