How to Validate Data in MySQL

Galaxy Glossary

How do I validate data in MySQL?

Validate data in MySQL by adding constraints and triggers that block invalid inserts or updates at the database layer.

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Description

Example H2

Example H3

Why validate data inside MySQL?

Placing validation in the database guarantees every client, API, or script obeys the same rules. This prevents bad rows, simplifies code, and protects analytics.

Which built-in constraints stop bad data?

Use NOT NULL, UNIQUE, CHECK, and FOREIGN KEY constraints. They run automatically on every INSERT or UPDATE and throw an error when rules are broken.

How do CHECK constraints work?

CHECK evaluates a Boolean expression on each row. If the result is FALSE, MySQL rejects the statement. Since 8.0.16, CHECK is enforced by default and cannot be disabled.

When should I use triggers?

Use BEFORE triggers for multi-column or cross-table checks that constraints can’t express. Inside a trigger you can raise custom errors with SIGNAL.

What are best practices for data validation?

Define constraints when you create tables. Give each constraint a descriptive name, e.g., chk_products_price. Keep business rules in one place so future migrations are easier.

Can I add validation to an existing table?

Yes. Use ALTER TABLE to add or modify constraints without recreating the table. Always test on staging and watch for failing rows.

How do I test my validation?

Insert both valid and invalid sample rows. Confirm valid data commits and invalid attempts raise a clear error such as ERROR 3819 (HY000): Check constraint 'chk_price_positive' is violated.

Common Mistakes

Application-only validation: Client code can be bypassed; enforce rules in MySQL too.

Omitted constraint names: Auto-generated names are hard to track; always specify your own.

Practical example recap

The syntax and example below show how an ecommerce database blocks negative prices, duplicate emails, and orphan orders.

Why How to Validate Data in MySQL is important

How to Validate Data in MySQL Example Usage


-- Enforce positive prices
ALTER TABLE Products
  ADD CONSTRAINT chk_products_price CHECK (price > 0);

-- Attempt to insert an invalid product
INSERT INTO Products (id, name, price, stock)
VALUES (101, 'Invalid Product', -5.00, 10);  -- Fails with ERROR 3819

How to Validate Data in MySQL Syntax


-- Add NOT NULL on email
ALTER TABLE Customers
  MODIFY email VARCHAR(255) NOT NULL;

-- Add UNIQUE for email addresses
ALTER TABLE Customers
  ADD CONSTRAINT uq_customers_email UNIQUE (email);

-- Add CHECK for positive product price
ALTER TABLE Products
  ADD CONSTRAINT chk_price_positive CHECK (price > 0);

-- Add FOREIGN KEY to link orders to customers
ALTER TABLE Orders
  ADD CONSTRAINT fk_orders_customer
  FOREIGN KEY (customer_id) REFERENCES Customers(id);

-- Trigger for positive quantity
DELIMITER //
CREATE TRIGGER trg_orderitems_qty
BEFORE INSERT ON OrderItems
FOR EACH ROW
BEGIN
  IF NEW.quantity <= 0 THEN
    SIGNAL SQLSTATE '45000'
      SET MESSAGE_TEXT = 'Quantity must be positive';
  END IF;
END//
DELIMITER ;

Common Mistakes

Relying solely on application code. If a different script bypasses the app, invalid data slips in. Always add constraints at the database layer.
Forgetting to name constraints. Auto-generated names like "CHECK_123" make debugging and migrations difficult. Provide clear names such as "chk_products_price".