sql constraints
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 databasesql 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
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