Are these FKs necessary -- and are they stopping me? - sql

Question

I'm having some difficulties with a database I'm creating for a summer camp, specifically with the PK and FK constraints. When I declare the FK constraint (e.g. FOREIGN KEY(PID) references Campers(CamperID)) I get an error running my code through pgAdmin (I'm using PostgreSQL). I understand that, for example, the Campers table is not yet created, and this is most likely part/all of the roadblock, however I feel like my FKs are still wrong somehow. To my understanding, a FK is a PK in another table -- but I feel like there is some redundancy or disconnect between my tables.
I've put the syntax for some of my CREATE statements below. I'm not sure if I'll get reprimanded for the quality of my (somewhat vague) question, but I feel a bit lost and would appreciate any help or advice. Thank you in advance!
DROP TABLE IF EXISTS People;
CREATE TABLE People (
PID VARCHAR(10) NOT NULL UNIQUE,
FName TEXT NOT NULL,
LName TEXT NOT NULL,
DOB DATE NOT NULL,
ArrivalDate DATE NOT NULL DEFAULT CURRENT_DATE,
DepartureDate DATE,
US_PhoneNum VARCHAR(11) NOT NULL,
StreetAddress VARCHAR(200) NOT NULL,
Sex GENDER NOT NULL,
ZIP VARCHAR(10) NOT NULL,
PRIMARY KEY(PID),
FOREIGN KEY(PID) REFERENCES Campers(CamperID),
FOREIGN KEY(PID) REFERENCES Counselors(CounselorID),
FOREIGN KEY(ZIP) REFERENCES Zip(ZIP)
);
DROP TABLE IF EXISTS Zip;
CREATE TABLE Zip (
ZIP VARCHAR(10) NOT NULL,
City TEXT NOT NULL,
State VARCHAR(2) NOT NULL,
PRIMARY KEY(ZIP)
);
DROP TABLE IF EXISTS Campers;
CREATE TABLE Campers (
CamperID VARCHAR(10) NOT NULL REFERENCES People(PID),
AgeGroup AGES NOT NULL,
CabinID VARCHAR(2) NOT NULL,
Bed BEDTYPES NOT NULL,
GroupID VARCHAR(3) NOT NULL,
PRIMARY KEY(CamperID),
FOREIGN KEY(CamperID) REFERENCES People(PID),
FOREIGN KEY(CabinID) REFERENCES Cabins(CabinID),
FOREIGN KEY(Bed) REFERENCES Beds(Bed),
FOREIGN KEY(GroupID) REFERENCES Groups(GroupID)
);
DROP TABLE IF EXISTS Counselors;
CREATE TABLE Counselors (
CounselorID VARCHAR(10) NOT NULL REFERENCES People(PID),
GroupID VARCHAR(3) NOT NULL,
CabinID VARCHAR(2) NOT NULL,
PRIMARY KEY(CounselorID),
FOREIGN KEY(GroupID) REFERENCES Groups(GroupID),
FOREIGN KEY(CabinID) REFERENCES Cabins(CabinID)
);
ERROR message for further clarification:
ERROR: relation "campers" does not exist
********** Error **********
ERROR: relation "campers" does not exist
SQL state: 42P01
There are more tables (obviously) which I can provide the create statements for, if needed.

Answer 1

You should really start here: Foreign key.
In the context of relational databases, a foreign key is a field (or
collection of fields) in one table that uniquely identifies a row of
another table.
What you are trying to do in your script is to create a circular link between People, Campers and Counselors. Having a Primary Key field also a Foreign Key mandates that IDs across all referenced tables are identical.
... and to create a Foreign Key the referenced table must already exist in the database. So you should start with the table that does not have any Foreign Keys and create tables that reference only those tables created previously. Alternatively you can create all tables without Foreign Keys and add them later, when all the tables are present.
... and to answer the question, Foreign Keys are never necessary, but they might help.