제약조건 Constraints

PRIMARY KEY (기본키)

CREATE TABLE users (
    id INT PRIMARY KEY,
    username VARCHAR(50)
);

-- 복합 키
CREATE TABLE enrollment (
    student_id INT,
    course_id INT,
    PRIMARY KEY (student_id, course_id)
);

FOREIGN KEY (외래키)

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    FOREIGN KEY (customer_id) REFERENCES customers(id)
        ON DELETE CASCADE
        ON UPDATE CASCADE
);

ON DELETE/UPDATE 옵션

CASCADE: 참조되는 행 삭제/수정 시 함께 삭제/수정
SET NULL: 참조되는 행 삭제 시 NULL로 설정
NO ACTION: 참조되는 행 삭제/수정 불가
RESTRICT: NO ACTION과 동일

UNIQUE (중복 방지)

CREATE TABLE users (
    id INT PRIMARY KEY,
    email VARCHAR(100) UNIQUE,
    phone VARCHAR(20) UNIQUE
);

NOT NULL (NULL 방지)

CREATE TABLE products (
    id INT PRIMARY KEY,
    name VARCHAR(100) NOT NULL,
    price DECIMAL(10, 2) NOT NULL
);

CHECK (조건 검증)

CREATE TABLE employees (
    id INT PRIMARY KEY,
    age INT CHECK (age >= 18),
    salary DECIMAL(10, 2) CHECK (salary > 0)
);

DEFAULT (기본값)

CREATE TABLE orders (
    id INT PRIMARY KEY,
    status VARCHAR(20) DEFAULT 'pending',
    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
);