[Solved] You have this SQL statement:ALTER TABLE Orders ADD FOREIGN KEY (CustomerID) REFERENCES Customers(CustomerID) ON DELETE SET NULL;But when you try to add it, you get 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) ON DELETE SET NULL;

But when you try to add it, you get an error. What is a likely cause?

AThe Customers table does not have a primary key

BThe CustomerID column in Orders is NOT NULL

CThe syntax for ON DELETE SET NULL is incorrect

DForeign keys cannot be added with ALTER TABLE

Step-by-Step Solution

Solution:

Step 1: Understand ON DELETE SET NULL requirement
The foreign key column must allow NULL to set it to NULL on delete.
Step 2: Check column nullability
If CustomerID is NOT NULL, setting NULL on delete causes error.
Final Answer:
The CustomerID column in Orders is NOT NULL -> Option B
Quick Check:
ON DELETE SET NULL requires nullable foreign key column [OK]

Quick Trick: ON DELETE SET NULL needs nullable foreign key column [OK]

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