1 PASSTHRU ( Expression
2? 
2.0? TO DatabaseReference
2.1? VALUES (
2.1.1+ ,
2.1.1 Expression
2.1.2 )
2  (1)  ,
2.1+ ,
2.1 Expression
2.1.1 )

WHERE

1 DatabaseReference =   Database . DataSourceClause

1 DataSourceClause =  
2 DataSourceName
2 { DataSourceExpression }