REFERENCING ISSUE - sql

Question

CREATE TABLE Loan
(
Customerid Char(9) NOT NULL,
EquipmentCode Char(5) NOT NULL,
StartDate DateTime NOT NULL,
EndDate DateTime NULL,
CONSTRAINT CHK_ID
CHECK (Customerid LIKE '[ST][0-9][0-9][0-9][0-9][0-9][0-9][0-9][A-Z]'),
CONSTRAINT CHK_Date CHECK (EndDate >= StartDate),
CONSTRAINT Loan_PK PRIMARY KEY(Customerid),
CONSTRAINT CUST_FK
FOREIGN KEY(Customerid) REFERENCES CUST(CustomerID)
ON UPDATE CASCADE
ON DELETE CASCADE
);
CREATE TABLE EQUIPMENT
(
EquipmentCode CHAR(5) NOT NULL,
EquipmentName VARCHAR(50) NOT NULL,
Description VARCHAR(255) NULL,
RentalRatePerDay DECIMAL(4,2) NOT NULL,
CONSTRAINT EQP_PK PRIMARY KEY(EquipmentCode),
CONSTRAINT CHK_Rate CHECK (RentalRatePerDay BETWEEN 4 AND 50),
CONSTRAINT LOAN_FK
FOREIGN KEY(EquipmentCode) REFERENCES Loan(EquipmentCode)
ON UPDATE CASCADE
ON DELETE CASCADE
);
I was able to reference earlier on in the above loan.table but for the equipment table it states
Msg 1776, Level 16, State 0, Line 49
There are no primary or candidate keys in the referenced table 'Loan' that match the referencing column list in the foreign key 'LOAN_FK'.
Msg 1750, Level 16, State 1, Line 49
Could not create constraint or index. See previous errors.
Please advise.

Answer 1

When we create a ForeignKey on a dependent table, it MUST refer back to the PrimaryKey (or a Unique Key see: https://stackoverflow.com/a/18435114/1690217) on the principal table.
In your case, EQUIPMENT is the principal end of the relationship, and Loan is the dependent. What this means is that the FK needs to be on the Loan table instead, so you should have this:
CREATE TABLE EQUIPMENT
(
EquipmentCode CHAR(5) NOT NULL,
EquipmentName VARCHAR(50) NOT NULL,
Description VARCHAR(255) NULL,
RentalRatePerDay DECIMAL(4,2) NOT NULL,
CONSTRAINT EQP_PK PRIMARY KEY(EquipmentCode),
CONSTRAINT CHK_Rate CHECK (RentalRatePerDay BETWEEN 4 AND 50),
);
CREATE TABLE Loan
(
Customerid Char(9) NOT NULL,
EquipmentCode Char(5) NOT NULL,
StartDate DateTime NOT NULL,
EndDate DateTime NULL,
CONSTRAINT CHK_ID
CHECK (Customerid LIKE '[ST][0-9][0-9][0-9][0-9][0-9][0-9][0-9][A-Z]'),
CONSTRAINT CHK_Date CHECK (EndDate >= StartDate),
CONSTRAINT Loan_PK PRIMARY KEY(Customerid),
CONSTRAINT CUST_FK
FOREIGN KEY(Customerid) REFERENCES CUST(CustomerID)
ON UPDATE CASCADE
ON DELETE CASCADE,
CONSTRAINT EQUIPMENT_FK
FOREIGN KEY(EquipmentCode) REFERENCES EQUIPMENT(EquipmentCode)
ON UPDATE CASCADE
ON DELETE CASCADE
);

Answer 2

A foreign key references a primary key on another table. You have the FK on the wrong table