pdo cannot accept multiple queries

multiple querries dont work for newly created forms.
for example, in onAfterUpdate event, i wrote "$update_sql = “SET @csum := 0; update ledger_main SET rt2 = (@csum := @csum + transaction_amount_debited_or_credited) where …”. As there are multiple querries here (NOTE THE SEMI-COLON AFTER 0), sc stops such multiple querries to be executed.

please help