[Solved] You try to create a foreign key constraint with:ALTER TABLE Orders ADD FOREIGN KEY (CustID) REFERENCES Customers(CustomerID);But get an error: "Cannot add foreign key constraint.... | SQL

SQL - Table Constraints

You try to create a foreign key constraint with:
ALTER TABLE Orders ADD FOREIGN KEY (CustID) REFERENCES Customers(CustomerID);
But get an error: "Cannot add foreign key constraint." What is a likely cause?

AThe Customers table does not have a primary key.

BThe Orders table already has a foreign key.

CThe CustID column data type does not match Customers.CustomerID.

DThe Orders table has no rows.

Step-by-Step Solution

Solution:

Step 1: Check foreign key requirements
Foreign key and referenced column must have matching data types.
Step 2: Analyze error cause
If data types differ, the constraint cannot be created, causing the error.
Final Answer:
The CustID column data type does not match Customers.CustomerID. -> Option C
Quick Check:
Data type mismatch = foreign key creation error [OK]

Quick Trick: Foreign key and referenced key must share data type [OK]

Common Mistakes:

MISTAKES

Assuming multiple foreign keys cause error
Ignoring primary key existence in parent table
Thinking empty tables cause foreign key errors

Master "Table Constraints" in SQL

9 interactive learning modes - each teaches the same concept differently

Learn Why Deep Visual Try Challenge Project Recall Time

More SQL Quizzes

You try to create a foreign key constraint with:

Step 1: Check foreign key requirements

Step 2: Analyze error cause

Final Answer:

Quick Check:

Want More Practice?