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.