Pergunta de entrevista da empresa Cognizant
primary key and composite key and unique key
Resposta da entrevista
Sigiloso
10 de mar. de 2018
he UNIQUE constraint uniquely identifies each record in a database table. The UNIQUE and PRIMARY KEY constraints both provide a guarantee for uniqueness for a column or set of columns. A PRIMARY KEY constraint automatically has a UNIQUE constraint defined on it. Note that you can have many UNIQUE constraints per table, but only one PRIMARY KEY constraint per table.