Constraints in sql

RULES ABOUT THE COLUMNS IN TABLE
NOT NULL 	 # Ensures a column cannot have a NULL value
UNIQUE 		 # Ensures all values in a column are unique
PRIMARY KEY  # Identifies a record in a table, is NOT NULL & UNIQUE
FOREIGN KEY  # References a unique record from another table
CHECK		 # Ensures all column values satisfy a condition
DEFAULT		 # Set a default value for a column if none is entered
INDEX		 # Quick way of retrieving records from database

Sql constarint

It creates a new constraint on an existing table, which is used to specify
rules for any data in the table.
Example: Adds a new PRIMARY KEY constraint named ‘user’ on columns
ID and SURNAME.
ALTER TABLE users
ADD CONSTRAINT user PRIMARY KEY (ID, SURNAME);

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