CHECK constraints prevent invalid data from being inserted or updated by setting rules in table definitions. For example, to ensure that the price is non-negative, the employee is between 18-65 years old, and the salary is greater than zero, you can use CHECK(price >= 0), CHECK(age >= 18 AND age
When you want to make sure the data in your SQL tables stays clean and meaningful, CHECK constraints are a straightforward way to do it. They let you define rules right in the table definition so invalid data doesn't sneak in during inserts or updates.
What a CHECK constraint does
A CHECK constraint makes sure that values going into a column (or across multiple columns) meet certain conditions. If the condition returns false, the database blocks the operation — simple as that.
For example, if you have a "prices" table and you never want negative prices allowed, you can add:
CHECK (price >= 0)
This stops anyone from accidentally inserting -5 as a price. It's not just numbers either — you can use them for strings, dates, or even logic that spans multiple fields.
How to set one up
You can create a CHECK constraint when making a new table or later on using an ALTER statement.
At table creation time:
CREATE TABLE employees (
id INT PRIMARY KEY,
age INT CHECK (age >= 18 AND age <= 65),
salary DECIMAL CHECK (salary > 0)
);Here, we're ensuring that all employees are between 18 and 65 years old and that no zero or negative salaries get stored.
After the table exists:
ALTER TABLE employees ADD CONSTRAINT check_salary_positive CHECK (salary > 0);
Some databases like MySQL support CHECK constraints but may ignore them unless configured properly, so always double-check your DBMS behavior.
Common use cases people miss
There are a few places where CHECK constraints really shine but often go unused.
Validating status codes : If your app uses short codes like 'A', 'P', 'C' to mean active, pending, cancelled — you can enforce only those letters:
CHECK (status IN ('A', 'P', 'C'))
Date ranges : You might want start dates before end dates in events or contracts:
CHECK (start_date <= end_date)
Email formats : While full regex validation is usually better handled in code, basic checks like requiring an '@' symbol can be done:
CHECK (email LIKE '%@%')
Just keep in mind that complex validations (like checking against another table) usually need triggers or application-level logic instead.
A couple gotchas to remember
They don't apply retroactively : Adding a CHECK constraint won't validate existing rows. So if you're adding one to an existing table, make sure current data already follows the rule.
Case sensitivity depends on your system : For string comparisons, some databases treat 'A' and 'a' differently; others don't. Be aware of collation settings.
Use named constraints : It's easier to manage them later if you give them names like
check_age_rangerather than relying on auto-generated ones.
So if you're trying to prevent bad data at the source, CHECK constraints are worth using — they're fast, easy, and effective for many common scenarios.
Basically that's it.
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