ORA-00936: missing expression error when inserting values - sql

Question

I spend lot of time searching where i made the mistake but i was unable to find it when its going to insert the last record there is a error message showing "ORA-00936: missing expression" How to solve this please help me
create type pearson_types as object(
name varchar2(50),
sysID char(6)
)NOT FINAL;
create type doctor_types under pearson_types(
regNo char(10),
specialization varchar2(25)
)
create table doctor of doctor_types(
regNo primary key
)
create type hospVisits_types as object(
hosChg float,
vDate varchar2(20),
refDoc REF doctor_types,
docChg float
)
create type hospvisits_tbl_types as table of hospVisits_types
create type phone_arr as VARRAY(3) of char(10)
create type patient_types under pearson_types
(
id char(10),
dob varchar(20),
phone phone_arr,
hospVisits hospvisits_tbl_types
)
create table patients of patient_types(
id primary key
)nested table hospVisits store as Hospital_tables
alter table Hospital_tables add scope for (refDoc) is doctor
insert into doctor values ('Dr.k.perera','D001','1223441234','Gynecologist');
insert into doctor values ('Dr.p.weerasingha','D002','1234421131','Dermatalogist');
insert into doctor values ('Prof .S. Fernando','D003','2342111322','Pediatrician');
insert into doctor values ('Dr.k.Sathgunanathan','D004','2344114344','Pediatrician');
insert into patients values('Sampath Weerasingha','P001','732821122V','23-JAN-73',phone_arr('0332124222'),hospvisits_tbl_types(hospVisits_types(50.00,'24-MAY-06',select ref (a) from doctor a where a.regNo='1223441234',500.00)))

Answer 1

Add parentheses to SELECT statements inside a SQL statement:
insert into patients values(
'Sampath Weerasingha','P001','732821122V','23-JAN-73',phone_arr('0332124222'),
hospvisits_tbl_types(hospVisits_types(50.00,'24-MAY-06',
( -- ADD ME
select ref (a) from doctor a where a.regNo='1223441234'
) -- ADD ME
,500.00))
);