How to Validate Data in SQL Server

Why validate data in SQL Server?

Bad values break reports, cause application errors, and complicate audits. Adding validation at the database layer catches issues early and keeps every consumer of the data secure and consistent.

Which SQL Server features enforce validation?

SQL Server offers NOT NULL, UNIQUE, PRIMARY KEY, FOREIGN KEY, DEFAULT, and CHECK constraints plus rules in triggers, computed columns, and TRY_CONVERT/TRY_CAST functions.

When should I use CHECK constraints?

Use CHECK when a column must follow a fixed rule—numeric ranges, string patterns, or cross-column comparisons. They run fast, are declarative, and integrate with query plans.

How do I add validation while creating a table?

CREATE TABLE Orders (
id INT PRIMARY KEY,
customer_id INT NOT NULL,
order_date DATE NOT NULL,
total_amount DECIMAL(10,2) NOT NULL,
CHECK (total_amount >= 0),
FOREIGN KEY (customer_id) REFERENCES Customers(id)
);


The CHECK stops negative totals, while the FOREIGN KEY prevents missing customers.

How do I add validation to an existing table?

ALTER TABLE Products
ADD CONSTRAINT chk_price_nonnegative CHECK (price >= 0);


ALTER TABLE lets you append constraints without recreating the table.

How can TRY_CONVERT help during inserts?

INSERT INTO Orders(id, customer_id, order_date, total_amount)
VALUES (101, 5, TRY_CONVERT(date,'2024-04-31'), 250.00);


TRY_CONVERT returns NULL on invalid dates, allowing you to catch the error with a NOT NULL constraint or custom logic.

How do I validate complex rules across rows?

Use AFTER triggers when checks depend on other rows. Keep logic short and raise explicit errors.

Example trigger

CREATE TRIGGER trg_stock_check
ON OrderItems
AFTER INSERT, UPDATE
AS
BEGIN
IF EXISTS (
SELECT 1
FROM OrderItems oi
JOIN Products p ON p.id = oi.product_id
WHERE oi.quantity > p.stock
)
BEGIN
RAISERROR('Ordered quantity exceeds stock.',16,1);
ROLLBACK;
END
END;


The trigger cancels any order that oversells inventory.

Best practices for data validation

Prefer constraints over triggers, keep rules single-purpose, name constraints clearly, and add unit tests with tSQLt or similar frameworks.