Referential integrity enforcement in SQL - Step-by-Step Execution

Concept Flow - Referential integrity enforcement

Start: Insert/Update/Delete in Child Table

↓

Check Foreign Key Value Exists in Parent Table?

↓

Allow Operation

↓

Operation Done

When you change data in a table with a foreign key, the database checks if the related value exists in the parent table. If yes, it allows the change; if no, it rejects it to keep data correct.

Execution Sample

SQL

CREATE TABLE Departments (
  DeptID INT PRIMARY KEY,
  DeptName VARCHAR(50)
);

CREATE TABLE Employees (
  EmpID INT PRIMARY KEY,
  EmpName VARCHAR(50),
  DeptID INT,
  FOREIGN KEY (DeptID) REFERENCES Departments(DeptID)
);

INSERT INTO Departments VALUES (1, 'Sales');
INSERT INTO Employees VALUES (101, 'Alice', 1);

This code creates two tables with a foreign key from Employees to Departments, then inserts a department and an employee linked to it.

Execution Table

Step	Operation	Foreign Key Check	Result	Notes
1	Insert DeptID=1 into Departments	No FK check needed	Success	Parent table insert always allowed
2	Insert EmpID=101, DeptID=1 into Employees	Check DeptID=1 exists in Departments	Success	DeptID=1 found, insert allowed
3	Insert EmpID=102, DeptID=2 into Employees	Check DeptID=2 exists in Departments	Fail	DeptID=2 not found, insert rejected
4	Delete DeptID=1 from Departments	Check Employees referencing DeptID=1	Fail	Child rows exist, delete rejected
5	Update EmpID=101 DeptID to 3	Check DeptID=3 exists in Departments	Fail	DeptID=3 not found, update rejected
6	Update EmpID=101 DeptID to NULL	Depends on FK constraint (if allowed)	Success or Fail	If FK allows NULL, update allowed; else rejected

💡 Operations stop when foreign key checks fail to maintain referential integrity.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
Departments	empty	[(1, 'Sales')]	[(1, 'Sales')]	[(1, 'Sales')]	[(1, 'Sales')]	[(1, 'Sales')]	[(1, 'Sales')]
Employees	empty	empty	[(101, 'Alice', 1)]	unchanged	unchanged	unchanged	depends on NULL allowance

Key Moments - 3 Insights

Why does inserting an employee with DeptID=2 fail even if Employees table is empty?

Because DeptID=2 does not exist in Departments, the foreign key check fails (see execution_table step 3). The database rejects the insert to keep data consistent.

Why can't we delete a department if employees reference it?

Deleting a department with existing employees referencing it breaks referential integrity, so the database rejects the delete (see execution_table step 4).

What happens if we update an employee's DeptID to a value not in Departments?

The update is rejected because the new DeptID does not exist in the parent table, violating referential integrity (see execution_table step 5).

Visual Quiz - 3 Questions

Look at the execution_table, what happens when inserting an employee with DeptID=1 at step 2?

A Insert fails because DeptID=1 is missing

B Insert is delayed until DeptID=1 is added

C Insert succeeds because DeptID=1 exists

D Insert succeeds without checking DeptID

💡 Hint

Check the 'Foreign Key Check' and 'Result' columns at step 2 in execution_table

At which step does the database reject an operation due to missing DeptID in Departments?

A Step 3

B Step 1

C Step 4

D Step 2

💡 Hint

Look for 'Fail' results with missing DeptID in execution_table

If the foreign key allowed NULLs, what would happen at step 6 when updating DeptID to NULL?

A Update rejected because NULL is not allowed

B Update succeeds if FK allows NULL

C Update causes deletion of employee

D Update ignored by database

💡 Hint

See notes in step 6 of execution_table about NULL allowance

Concept Snapshot

Referential integrity means foreign keys must match existing parent keys.
When inserting or updating child rows, DB checks parent table for matching key.
If no match, operation is rejected to keep data consistent.
Deleting parent rows is blocked if children reference them.
NULL foreign keys may be allowed depending on constraint settings.

Full Transcript

Referential integrity enforcement ensures that foreign key values in a child table always match existing primary key values in a parent table. When you insert or update a row in the child table, the database checks if the foreign key value exists in the parent table. If it does, the operation proceeds; if not, the database rejects it to prevent broken links. Similarly, deleting a parent row is blocked if child rows reference it. This keeps data consistent and prevents orphaned records. Some foreign keys allow NULL values, which means the child row can have no parent reference if allowed by the constraint. The execution table shows examples of inserts, updates, and deletes with their foreign key checks and results, illustrating how referential integrity is enforced step-by-step.