$query->leftJoin('modTemplateVarResource', 'tv13', 'tv13.contentid = modResource.id'); //with a custom join-alias, you need to define a ON - statement
$query->where(array(
'modResource.parent' => 679,
'modResource.published' => 1,
'tv13.tmplvarid' => 13,
'tv13.value' => 603
));
what do you get with
$query->prepare();echo $query->toSql();
try to see what you get with this generated query in phpmyadmin
ERROR 1054 (42S22): Unknown column 'tv_value.resourceid' in 'on clause'
$query->leftJoin('modTemplateVarResource', 'tv_value', 'tv_value.contentid = modResource.id');$query->prepare();echo $query->toSql();