sql create table constraint code example
Example 1: constraints in sql
# The most common constraints are
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
Example 2: 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);