Why should you care?

Without CHECK (Price > 0), a bug can insert free products. Constraints catch errors at the database boundary.

See it live — copy this example

Run in SQL Server Management Studio (SSMS) or Azure Data Studio.

USE DataVerse;
IF OBJECT_ID(N'dbo.Accounts', N'U') IS NOT NULL DROP TABLE dbo.Accounts;
CREATE TABLE dbo.Accounts (
    AccountId INT IDENTITY(1,1) PRIMARY KEY,
    AccountNo VARCHAR(20) NOT NULL CONSTRAINT UQ_Accounts_AccountNo UNIQUE,
    Balance   DECIMAL(18,2) NOT NULL
        CONSTRAINT CK_Accounts_Balance CHECK (Balance >= 0),
    Status    VARCHAR(10) NOT NULL
        CONSTRAINT CK_Accounts_Status CHECK (Status IN ('Open','Closed'))
);
INSERT INTO dbo.Accounts (AccountNo, Balance, Status)
VALUES ('SB-1001', 5000.00, 'Open');
-- This should fail:
-- INSERT INTO dbo.Accounts (AccountNo, Balance, Status) VALUES ('SB-1002', -10, 'Open');

What happened?

• UNIQUE blocks duplicate account numbers.
• CHECK keeps Balance non-negative and Status in a small list.
• The commented INSERT shows what the engine will reject.

Practice next

1. Create dbo.Accounts and insert the good row.
2. Uncomment the negative Balance INSERT and run it.
3. Read the CHECK constraint error message.
4. Add DEFAULT ('Open') on Status and insert without Status.
5. Try two rows with the same AccountNo.

Remember

Constraints enforce data rules in the engine. PRIMARY KEY / UNIQUE / CHECK are day-one tools. Failed INSERT means the rule worked.

Ledger never goes negative

DataVerse banking module uses CHECK (Balance >= 0) on Accounts.

Outcome: A buggy transfer that overdrafts fails at INSERT/UPDATE time.