← VCE Spreadsheets, SQL & Visualisation

SQL constraints for validation

A constraint is a rule on a column. This CHECK rejects any record with an age outside 0 to 120. NOT NULL (must have a value) and UNIQUE (no repeats) are constraints too.

Part of: VCE Applied Computing - U1O1 - Data analysis — Module 47, VCE Data Analytics - U3O1 - Data Analytics — Module 52, 9-10 Digital Technologies: Acquiring, storing and validating data — Module 27, ACARA Digital Technologies Years 9–10 — Acquiring and Validating Data — Module 80, QCE Digital Solutions Unit 2 — Data-Driven Problems and Solution Requirements — Module 50, QCE Digital Solutions Unit 2 — Data and Programming Techniques — Module 82, QCE Digital Solutions Unit 2 — Prototype Data Solutions — Module 18, QCE Digital Solutions Unit 4 — Prototype Digital Data Exchanges — Module 63, WA Computer Science ATAR Year 11 — Data Management — Module 53

Understanding SQL constraints for validation in VCE Spreadsheets, SQL & Visualisation

What it does: UNIQUE means no value can repeat in that column, so a second record with the same email is rejected. CHECK with BETWEEN is a range check. Any mark below 0 or above 100 is rejected. CHECK is the constraint used for a range check. NOT NULL does not take a condition. The data type comes first, then the constraint: name TEXT NOT NULL. BETWEEN includes the start and end values, so 0 and 120 are both allowed. Constraints validate data on entry. They reject bad records rather than correcting them later. A CHECK constraint needs its rule inside brackets. Without them the database cannot read the rule.

Common mistakes: The condition inside a CHECK constraint must be wrapped in brackets. It should be CHECK (age >= 0). The presence check constraint is written NOT NULL, not NO NULL. It stops the column being left empty.

Class: _____________________ · Name: _____________________ · Date: _________

Free printable worksheet with full answer key — or add this module to a class and your students practise it interactively.

🖨 Print worksheet + answer key Add SQL constraints for validation to a class →

This module's key terms are also in a printable crossword (Modules 45–54) and word search (Modules 35–54).

Take this worksheet further — free with Cloodit

Each concept is reinforced from every angle — learned, then applied, then tested.

Learning

Duo

Which constraint makes sure a column can never be left empty (a presence check)?

UNIQUENOT NULL

Matches

name TEXT NULL NOT

↓

The words are in the wrong order. NOT NULL is the keyword pair and it should be written as name TEXT NOT NULL.

Applying

Meanings

Term

CHECK (age BETWEEN 0 AND 120)

Meaning

A constraint is a rule on a column. This CHECK rejects any record with an age outside 0 to 120. NOT NULL (must have a value) and UNIQUE (no repeats) are constraints too.

Practice

CREATE TABLE student (

Testing

Gaps

name TEXT ,

Check Answer

Spots

2 id INT UNIQUE,

3 age INT CHECK age >= 0,

Cloodit turns curriculum-aligned coding content into journeys students work through one biome at a time.

Students earn collectable badges

Over 100 of them, across seven natural biomes.

Study Buddy

Students build a real study resource

Turns their work into revision material ready for Flash Cards and Exam study.

Diamond Dash — one of Cloodit's live, competitive Sprint games

Students lock in with Sprints

Live, competitive practice for the start of the lesson or a teacher-led lesson finisher.

A Cloodit certificate of achievement, ready to personalise and print

Custom printable certificates

Honour your top performers, or encourage a student who's trying hard — ready to print in seconds.

Add to class →

Quick Questions

1. Which constraint makes sure a column can never be left empty (a presence check)?

A. NOT NULL ✓

B. UNIQUE

NOT NULL means the column must have a value. UNIQUE only stops repeated values.

2. Which constraint stops two records from having the same value in a column, such as two students with the same email?

A. UNIQUE ✓

B. NOT NULL

UNIQUE rejects repeats. NOT NULL only checks that a value is present.

3. Which is the correct way to make sure age is between 0 and 120?

A. CHECK (age BETWEEN 0 AND 120) ✓

B. NOT NULL (age BETWEEN 0 AND 120)

CHECK is the constraint used for a range check. NOT NULL does not take a condition.

4. A table has the rule CHECK (age BETWEEN 0 AND 120). What happens when you try to INSERT a record with age 150?

A. The database saves it and changes the age to 120

B. The database rejects the record ✓

A constraint stops a record that breaks the rule from being saved. The database does not fix the value for you.

5. Which type of validation check does CHECK (age BETWEEN 0 AND 120) perform?

A. A presence check

B. A range check ✓

It tests that the number sits within a set range. A presence check is done by NOT NULL.

Match the Pairs

name TEXT NOT NULL → A presence check: the database will not accept a record if the name field is left empty.

CHECK (age BETWEEN 0 AND 120) → A range check: the database rejects any record where age is below 0 or above 120.

A constraint that stops two students being saved with the same email address → email TEXT UNIQUE

A constraint that only allows a mark between 0 and 100 → CHECK (mark BETWEEN 0 AND 100)

name TEXT NULL NOT → The words are in the wrong order. NOT NULL is the keyword pair and it should be written as name TEXT NOT NULL.

CHECK age BETWEEN 0 AND 120 → The condition is missing its brackets. It should be CHECK (age BETWEEN 0 AND 120).

Key Term

CHECK (age BETWEEN 0 AND 120)

A constraint is a rule on a column. This CHECK rejects any record with an age outside 0 to 120. NOT NULL (must have a value) and UNIQUE (no repeats) are constraints too.

Fill in the Blanks

1. name TEXT NOT NULL,

Hint: Type the constraint that works as a presence check, so the column cannot be left empty.

2. email TEXT UNIQUE,

Hint: Type the constraint that stops two records from having the same value in this column.

3. age INTEGER CHECK (age BETWEEN 0 AND 120)

Hint: Type the operator that tests whether a value falls inside a range.

4. age INTEGER CHECK (age BETWEEN 0 AND 120)

Hint: Type the constraint keyword that applies a range rule to the column.

5. CREATE TABLE pupil ( age INTEGER CHECK (age BETWEEN 0 AND 120) );

Hint: Type the name of the column that the range rule is checking.

Spot the Error

Question 1

A CREATE TABLE student (

B id INT UNIQUE,

C age INT CHECK age >= 0,

D name TEXT NOT NULL

E );

Line C — The condition inside a CHECK constraint must be wrapped in brackets. It should be CHECK (age >= 0).

Question 2

A CREATE TABLE pet (

B name TEXT NO NULL,

C id INT UNIQUE

D );

Line B — The presence check constraint is written NOT NULL, not NO NULL. It stops the column being left empty.

Question 3

A CREATE TABLE player (

B id INT PRIMARY KEY,

C age INT NOT NULL,

D CHECK (age BETWEEN 0, 120)

E );

Line D — BETWEEN needs the keyword AND between the two limits, not a comma. It should be CHECK (age BETWEEN 0 AND 120).

Question 4

A CREATE TABLE member (

B email TEXT UNIQUE

C age INT NOT NULL

D );

Line B — There is a comma missing at the end of the line. Column definitions in CREATE TABLE are separated by commas, so it should be email TEXT UNIQUE,

Question 5

A CREATE TABLE ticket (

B id INT UNIQUE,

C qty INT CHECK (qty < 0),

D seat TEXT NOT NULL

E );

Line C — The rule is the wrong way round. It would only accept negative quantities. To accept only positive quantities it should be CHECK (qty > 0).

Student with a class code? Join your class →