what are the constraints in sql
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 databasesql constraints
NOT NULL - Ensures that a column cannot have a NULL value
UNIQUE - Ensures that all values in a column are different
PRIMARY KEY - A combination of a NOT NULL and UNIQUE. Uniquely identifies each row in a table
CREATE TABLE table_name (
column1 datatype constraint,
column2 datatype constraint,
column3 datatype constraint,
....
);
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255) NOT NULL,
Age int
);
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
UNIQUE (ID)
);
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int,
PRIMARY KEY (ID)
);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 databaseconstraint sql
create table etc(
numeric (4) anything constraint Gives_name_to_a_restriction primary key);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);Copyright © 2021 Codeinu
Forgot your account's password or having trouble logging into your Account? Don't worry, we'll help you to get back your account. Enter your email address and we'll send you a recovery link to reset your password. If you are experiencing problems resetting your password contact us
