Structured Query Language (Basics 2 video)

                    BASICS 2

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
FOREIGN KEY = Prevents actions that would destroy links

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SELECT * FROM Persons;
CREATE TABLE Persons (
ID int NOT NULL PRIMARY KEY,
LastName varchar(255) NOT NULL,
FirstName varchar(255),
Age int
);

INSERT INTO Persons(ID,LastName,FirstName,Age) VALUES(
001,
'Abdul Rahman',
'',
24
);

INSERT INTO Persons(ID,LastName,FirstName,Age) VALUES(
002,
'Abdul Kahar',
NULL,
24
);

CREATE TABLE Class(
ID INT FOREIGN KEY REFERENCES Persons(ID),
Present VARCHAR(3) NOT NULL,
Ddate VARCHAR(12) NOT NULL
);

INSERT INTO Class(ID,Present,Ddate) VALUES(
002,
'Yes',
'13/12/22'
);

SELECT * FROM Class

ALTER TABLE Class ADD signed VARCHAR(3)

ALTER TABLE friends ADD PRIMARY KEY (contact)

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