plsql - #PLSQL_Trigger duplicate customer doesnt work -

homework requirement: create trigger account must belong 1 , 1 customer.

my code shown below doesn't work. error mesg: quoted string not terminated. please help.

create or replace trigger dupcust before insert or update on account each row declare     v_ctn number; begin     select count(account.cname) v_ctn account a#=:new.a#;     if v_ctn>0           raise_application_error (-20107, 'acct can belong 1 customer');     end if; end; 

test code:

update account set account.cname =’cook’ account.a# = ‘1111’; 

when copied query, failed due quotes around string cook , 1111. apparently used wrong quote character, corrected quotes , when run query, trigger generates mutation error, answer can can found here:

plsql trigger error. "is mutating, trigger/function may not see it" ora-04091

basically reading table updating, causes mutation, can't that.

a compound trigger, if written correctly, job, example here:

oracle trigger after insert or delete