CHECK constraint in SQL - Step-by-Step Execution

Concept Flow - CHECK constraint

Start Table Creation

↓

Define Columns

↓

Add CHECK Constraint

↓

Insert/Update Data

↓

Check Condition on Data

↓

Allow Data

↓

End

When creating or modifying a table, a CHECK constraint sets a rule. When data is added or changed, the database checks if the data meets the rule. If yes, it accepts the data; if no, it rejects it.

Execution Sample

SQL

CREATE TABLE Employees (
  ID INT,
  Age INT CHECK (Age >= 18)
);

INSERT INTO Employees VALUES (1, 20);
INSERT INTO Employees VALUES (2, 16);

This code creates a table with a rule that Age must be 18 or older. It tries to add two employees, one with Age 20 (allowed) and one with Age 16 (rejected).

Execution Table

Step	Action	Data Checked	Condition	Result	Notes
1	Create table Employees	N/A	N/A	Success	Table created with CHECK constraint Age >= 18
2	Insert (1, 20)	Age=20	20 >= 18	Allowed	Data meets CHECK constraint
3	Insert (2, 16)	Age=16	16 >= 18	Rejected	Data violates CHECK constraint

💡 Insertion stops when data violates CHECK constraint (Age < 18)

Variable Tracker

Variable	Start	After Step 2	After Step 3
Employees Table Rows	Empty	(1, 20)	(1, 20) only; (2, 16) rejected

Key Moments - 2 Insights

Why was the second insert rejected even though the table exists?

Because the CHECK constraint requires Age to be 18 or more. The second insert has Age=16, which fails the condition (see execution_table row 3).

Does the CHECK constraint apply only when inserting data?

No, it applies both when inserting new data and when updating existing data to ensure the condition is always true.

Visual Quiz - 3 Questions

Look at the execution table, what happens at Step 3?

A The data is inserted successfully

B The data is rejected due to CHECK constraint

C The table is dropped

D The condition is ignored

💡 Hint

Check the 'Result' column in execution_table row 3

According to variable_tracker, how many rows are in the table after Step 3?

A 0 rows

B 2 rows

C 1 row

D 3 rows

💡 Hint

Look at the 'Employees Table Rows' after Step 3 in variable_tracker

If the CHECK constraint was changed to Age >= 16, what would happen at Step 3?

A The insert would be allowed

B The insert would still be rejected

C The table would be deleted

D The first insert would fail

💡 Hint

Refer to the condition column in execution_table and imagine changing 18 to 16

Concept Snapshot

CHECK constraint sets a rule on table columns.
It validates data on insert or update.
If data breaks the rule, database rejects it.
Syntax example: Age INT CHECK (Age >= 18).
Useful for data integrity and simple validations.

Full Transcript

The CHECK constraint is a rule added to a table column to ensure data meets certain conditions. When you insert or update data, the database checks if the data satisfies the condition. If it does, the data is accepted; if not, it is rejected. For example, a CHECK constraint on Age >= 18 means only ages 18 or older are allowed. In the example, inserting Age 20 succeeds, but Age 16 fails and is rejected. This helps keep data clean and valid automatically.

Practice

1. What is the main purpose of a CHECK constraint in SQL?

easy

A. To automatically generate unique IDs

B. To create a backup of the table data

C. To speed up query performance

D. To enforce rules on column values to prevent invalid data

Solution

1. Step 1: Understand what a CHECK constraint does

   A CHECK constraint sets a rule that data must follow when inserted or updated in a table.

2. Step 2: Identify the purpose from the options

   Only To enforce rules on column values to prevent invalid data describes enforcing rules on data to prevent invalid entries.

3. Final Answer:

   To enforce rules on column values to prevent invalid data -> Option D

4. Quick Check:

   CHECK constraint = enforce data rules [OK]

Hint: CHECK constraints prevent bad data from entering tables [OK]

Common Mistakes:

• Confusing CHECK with indexing or keys
• Thinking CHECK creates backups
• Assuming CHECK improves speed

2. Which of the following is the correct syntax to add a CHECK constraint on a column age to allow only values greater than 18?

easy

A. ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18);

B. ALTER TABLE users CHECK (age > 18);

C. ALTER TABLE users ADD CHECK age > 18;

D. ALTER TABLE users ADD CONSTRAINT chk_age CHECK age > 18;

Solution

1. Step 1: Recall correct syntax for adding CHECK constraint

   The correct syntax uses ADD CONSTRAINT with a name, then CHECK with parentheses around the condition.

2. Step 2: Compare options to syntax

   ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18); matches the syntax: ADD CONSTRAINT name CHECK (condition). Options A and C miss the constraint name or parentheses; D misses parentheses.

3. Final Answer:

   ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18); -> Option A

4. Quick Check:

   ADD CONSTRAINT name CHECK (condition) = ALTER TABLE users ADD CONSTRAINT chk_age CHECK (age > 18); [OK]

Hint: Use ADD CONSTRAINT name CHECK (condition) syntax [OK]

Common Mistakes:

• Omitting parentheses around condition
• Not naming the constraint
• Writing condition outside CHECK()

3. Given the table creation:

CREATE TABLE products (
  id INT,
  price DECIMAL(5,2),
  CONSTRAINT chk_price CHECK (price >= 0)
);

What happens if you run:

INSERT INTO products (id, price) VALUES (1, -10.00);

medium

A. The row is inserted successfully

B. The price is automatically set to 0

C. The insert fails due to CHECK constraint violation

D. The insert causes a syntax error

Solution

1. Step 1: Understand the CHECK constraint on price

   The constraint requires price to be greater than or equal to 0.

2. Step 2: Analyze the insert statement

   The insert tries to add price = -10.00, which violates the CHECK condition.

3. Final Answer:

   The insert fails due to CHECK constraint violation -> Option C

4. Quick Check:

   price < 0 violates CHECK = insert fails [OK]

Hint: CHECK rejects rows violating the condition [OK]

Common Mistakes:

• Assuming negative values are accepted
• Thinking CHECK auto-corrects values
• Confusing CHECK violation with syntax error

4. You try to create a table with this statement:

CREATE TABLE employees (
  id INT,
  salary INT CHECK salary > 0
);

But get a syntax error. What is the likely fix?

medium

A. Add a semicolon after salary

B. Add parentheses around the CHECK condition: CHECK (salary > 0)

C. Change salary to VARCHAR

D. Remove the CHECK constraint entirely

Solution

1. Step 1: Identify syntax for inline CHECK constraints

   Inline CHECK constraints require the condition to be inside parentheses.

2. Step 2: Check the given statement

   The statement misses parentheses around salary > 0, causing syntax error.

3. Final Answer:

   Add parentheses around the CHECK condition: CHECK (salary > 0) -> Option B

4. Quick Check:

   Inline CHECK needs parentheses [OK]

Hint: Always use parentheses around CHECK conditions [OK]

Common Mistakes:

• Omitting parentheses in inline CHECK
• Changing data type unnecessarily
• Ignoring syntax error details

5. You want to ensure a discount column in orders table is between 0 and 50 inclusive. Which CHECK constraint correctly enforces this?

hard

A. CHECK (discount >= 0 AND discount <= 50)

B. CHECK (discount > 0 AND discount < 50)

C. CHECK (discount BETWEEN 1 AND 50)

D. CHECK (discount != 0 OR discount != 50)

Solution

1. Step 1: Understand the required range including boundaries

   The discount must be at least 0 and at most 50, so boundaries are included.

2. Step 2: Evaluate each option

   B: CHECK (discount > 0 AND discount < 50) excludes 0 and 50 (strict inequalities). A: CHECK (discount >= 0 AND discount <= 50) includes boundaries with >= and <=. C: CHECK (discount BETWEEN 1 AND 50) excludes 0 (>=1 <=50). D: CHECK (discount != 0 OR discount != 50) is logically incorrect.

3. Final Answer:

   CHECK (discount >= 0 AND discount <= 50) -> Option A

4. Quick Check:

   Inclusive range needs >= and <= [OK]

Hint: Use >= and <= for inclusive CHECK ranges [OK]

Common Mistakes:

• Using > and < excludes boundary values
• Using incorrect bounds with BETWEEN
• Logical errors with OR instead of AND