NOT NULL constraint enforces a column to NOT accept NULL values.
CREATE TABLE User (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255) NOT NULL,
Age int
);
UNIQUE constraint ensures that all values in a column are different.
CREATE TABLE User (
ID int NOT NULL UNIQUE,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
);
PRIMARY KEY constraint uniquely identifies each record in a table.
CREATE TABLE User (
ID int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
);
FOREIGN KEY constraint is used to prevent actions that would destroy links between tables.
CREATE TABLE Orders (
OrderID int NOT NULL PRIMARY KEY,
OrderNumber int NOT NULL,
UserID int FOREIGN KEY REFERENCES User(UserID)
);
CHECK constraint is used to limit the value range that can be placed in a column.
CREATE TABLE Persons (
ID int NOT NULL,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int CHECK (Age>=18)
);
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