NOT NULL, UNIQUE, CHECK and DEFAULT.
✍️ 3 exercises + code challenge in this lessonConstraints are rules that keep data valid. The database rejects any change that breaks them.
NOT NULL: the column must have a value.UNIQUE: all values must be different.PRIMARY KEY: uniquely identifies each row (NOT NULL + UNIQUE).FOREIGN KEY: must match a key in another table.CHECK: values must satisfy a condition.DEFAULT: the value used when none is given.Mastering SQL Constraints helps you write clean and reliable SQL code. Follow standard conventions, test edge cases, and keep logic simple. Try running the code example below and complete the exercises to build confidence.
CREATE TABLE Accounts (
AccountID INTEGER PRIMARY KEY,
Username TEXT NOT NULL UNIQUE,
Age INTEGER CHECK (Age >= 18),
Status TEXT DEFAULT 'active'
);
INSERT INTO Accounts (Username, Age) VALUES ('ali', 25);
SELECT * FROM Accounts;
-- This breaks the CHECK constraint, so it fails:
INSERT INTO Accounts (Username, Age) VALUES ('kid', 12);Check what you learned in “SQL Constraints”.
Every username must be different.
Username TEXT NOT NULL
Which constraint gives a column a value when none is provided?
Create table Members (ID INTEGER PRIMARY KEY, Status TEXT DEFAULT 'active'), insert a row with only ID 1, then select Status.
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