[Solved] You have this SQL statement:ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);But it returns an error. What is a likely cause? | SQL

SQL - Table Relationships

You have this SQL statement:
ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID);
But it returns an error. What is a likely cause?

ACustomerID column is missing in Orders table.

BThe Orders table already has a foreign key on CustomerID.

CThe syntax is missing a semicolon.

DThe Customers table does not have CustomerID as a primary or unique key.

Step-by-Step Solution

Solution:

Step 1: Check foreign key requirements
The referenced column must be a primary key or unique in the referenced table.
Step 2: Identify common error causes
If Customers.CustomerID is not primary or unique, the foreign key cannot be created, causing an error.
Final Answer:
The Customers table does not have CustomerID as a primary or unique key. -> Option D
Quick Check:
Referenced key must be unique or primary = The Customers table does not have CustomerID as a primary or unique key. [OK]

Quick Trick: Referenced key must be unique or primary [OK]

Common Mistakes:

MISTAKES

Ignoring uniqueness requirement on referenced key
Assuming syntax error without checking keys
Overlooking missing columns in tables

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You have this SQL statement:

Step 1: Check foreign key requirements

Step 2: Identify common error causes

Final Answer:

Quick Check:

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