Αποτελέσματα Αναζήτησης
14 Νοε 2011 · The main difference between PRIMARY KEY and UNIQUE KEY is that the PRIMARY KEY never takes NULL value while a UNIQUE KEY may take NULL value. Also, there can be only one PRIMARY KEY in a table while UNIQUE KEY may be more than one.
30 Αυγ 2024 · Every table must have at least a single candidate key. A table can have multiple candidate keys but only one primary key. The value of the Candidate Key is unique and may be null for a tuple. There can be more than one candidate key in a relationship. Example: STUD_NO is the candidate key for relation STUDENT. Table STUDENT.
Nulls can appear in columns of any data type that are not restricted by NOT NULL or PRIMARY KEY integrity constraints. Use a null when the actual value is not known or when a value would not be meaningful. Oracle Database treats a character value with a length of zero as null.
28 Ιουν 2024 · An artificial key which aims to uniquely identify each record is called a surrogate key; Primary Key never accept null values while a foreign key may accept multiple null values.
16 Νοε 2023 · Candidate keys play an essential role in Database Management Systems (DBMS) by ensuring data integrity and efficient retrieval. A candidate key refers to a set of attributes that can uniquely identify each record in a table.
2 Ιουλ 2024 · Keys are the entity set that are used to identify an entity within its entity set uniquely. An entity set can contain multiple keys, bit out of them one key will be primary key. A primary key is always unique, it does not contain any null value in table. Example: It is now acceptable because all rows must be unique.
28 Ιουν 2024 · Relation key – Every row has one, two or multiple attributes, which is called relation key. Attribute domain – Every attribute has some pre-defined value and scope which is known as attribute domain
