NOT NULL, UNIQUE, CHECK and DEFAULT

Column-Level Constraints

Each Constraint

CREATE TABLE students (
  roll_no INT PRIMARY KEY,
  name VARCHAR(50) NOT NULL,          -- must have a value
  email VARCHAR(60) UNIQUE,           -- no duplicates
  marks INT CHECK (marks BETWEEN 0 AND 100),  -- valid range
  status VARCHAR(10) DEFAULT 'active' -- auto value
);

Quick Meaning

Summary

• NOT NULL = required; UNIQUE = no duplicates.
• CHECK = condition must hold; DEFAULT = auto value.

Column-Level Constraints

हर Constraint

CREATE TABLE students (
  roll_no INT PRIMARY KEY,
  name VARCHAR(50) NOT NULL,          -- value होनी चाहिए
  email VARCHAR(60) UNIQUE,           -- कोई duplicate नहीं
  marks INT CHECK (marks BETWEEN 0 AND 100),  -- valid range
  status VARCHAR(10) DEFAULT 'active' -- auto value
);

तुरंत मतलब

सारांश

• NOT NULL = ज़रूरी; UNIQUE = कोई duplicate नहीं।
• CHECK = condition पूरी हो; DEFAULT = auto value।