sql - No unique index found for the referenced field of the primary table -

when run booking table, show message "no unique index found referenced field of primary table".

create table customer

create table customer ( custid               integer not null, fname             char(18) not null, lname            char(18) not null, street                char(6) not null, city                 char(18) not null, province             char(8) not null, country              char(8) not null, postcode                char(6) not null, gender char(6) not null, primary key (custid) ); 

create table booking

create table booking  ( bkgno               integer not null, custid               integer not null, fno                    integer not null,    statusid           char(3) not null, classid             char(4) not null, orig                  char(18) not null, dest                   char(18), depttime           date not null, arrtime            date not null, bkgcity             char(18) not null, paidby               char(18) not null, fprice               currency not null, totprice           currency not null, paidamt            currency not null, bal                     currency not null, bkgdate            date not null, primary key (bkgno), index (orig,dest,depttime, arrtime), index (custid), foreign key (custid) references customer, foreign key (fno) references flight_availability(fno), foreign key(dest) references flight_availability(dest), foreign key(depttime) references flight_availability(depttime), foreign key(arrtime) references flight_availability(arrtime), foreign key(dest,depttime,arrtime) references flight_availability(dest, depttime, arrtime), foreign key (orig) references airport(airportcd) ); 

create table flight availability

create table flight_availability  ( fno                  integer not null, orig               char(18) not null, dest                char(18), depttime        date not null, arrtime        date not null, flength         integer not null, primary key (fno,orig,dest,depttime,arrtime) ); 

create tableairport

create table airport  ( airportcd        char(18) not null, cityid                 char(18) not null, airportnm       char(18) not null, airporttax       currency,        primary key (airportcd) ); 

this part causes errors:

foreign key (fno) references flight_availability(fno), foreign key(dest) references flight_availability(dest), foreign key(depttime) references flight_availability(depttime), foreign key(arrtime) references flight_availability(arrtime), foreign key(dest,depttime,arrtime) references flight_availability(dest, depttime, arrtime) 

a foreign key constraint (also called referential integrity constraint) designates column foreign key , establishes relationship between foreign key , specified primary or unique key, called referenced key. composite foreign key designates combination of columns foreign key.

so referenced key should either unique or primary key.

for example:

foreign key (fno) references flight_availability(fno) 

referenced key = fno not unique , not primary key. although fno part of composite primary key (fno,orig,dest,depttime,arrtime), it's not unique (the combination of columns of primary key unique).

p.s. don't need many columns in primary key in flight_availability table. may create composite unique index on columns (if it's required) , create not null constraint on each column. in case you'll have same checks in current composit primary key. don't think need this.