I vincoli NOT NULL e UNIQUE

Introduzione ai database relazionali in SQL

Timo Grossenbacher

Data Journalist

Il vincolo NOT NULL

CREATE TABLE students (
 ssn integer not null,
 lastname varchar(64) not null,
 home_phone integer,
 office_phone integer
);

NULL != NULL

Quando crei una tabella...

CREATE TABLE students (
 ssn integer not null,
 lastname varchar(64) not null,
 home_phone integer,
 office_phone integer
);

Dopo aver creato la tabella...

ALTER TABLE students 
ALTER COLUMN home_phone 
SET NOT NULL;

ALTER TABLE students 
ALTER COLUMN ssn 
DROP NOT NULL;

CREATE TABLE table_name (
 column_name UNIQUE
);

ALTER TABLE table_name
ADD CONSTRAINT some_name UNIQUE(column_name);

Introduzione ai database relazionali in SQL