I have tried:
$previousValue = get_record_select("myblock_root_record", "timestamp={$yesterday} AND id = 1", "*");
$previousValue = $previousValue->previousrootval;
e.g previousrootval in the table could be the string "languages"
Thanks
The most likely thing is that no records match your query.
In particular, if you are selecting on id, there is really no point including anything else in the query conditions, because id is a primary key.