What are key attributes?

Inhaltsverzeichnis:

1. What are key attributes?
2. What are the types of attributes?
3. What is attribute and its types?
4. What type of attribute is age?
5. What is descriptive attribute?
6. What is a single valued attribute?
7. What is composite attribute?
8. Which of the following is a single valued attribute?
9. Which of the following can be multi valued attribute?
10. Which of the following is a derived attribute?
11. Which one of the following attribute can be taken as a primary key?
12. What is primary key and example?
13. Does every table need a foreign key?
14. Can foreign key be null?
15. Is foreign key unique?
16. Can foreign key be not unique?
17. Does a foreign key have to be unique?
18. Can a foreign key not be a primary key?
19. Is foreign key can be primary key?
20. Is unique key same as primary key?
21. Is a primary key unique?
22. What is unique key example?
23. What is difference between primary key and index?
24. Is primary key an index?
25. Which is faster primary key or index?
26. What is foreign key SQL?

What are key attributes?

A key attribute is the unique characteristic of the entity. Name and hire date are attributes of the entity Employee. ...

What are the types of attributes?

Types of Attributes-

• Simple attributes.
• Composite attributes.
• Single valued attributes.
• Multi valued attributes.
• Derived attributes.
• Key attributes.

What is attribute and its types?

An attribute is a property or characteristic of an entity. An entity may contain any number of attributes. One of the attributes is considered as the primary key. ... There are five such types of attributes: Simple, Composite, Single-valued, Multi-valued, and Derived attribute.

What type of attribute is age?

Example: Class, Section, Age, Name etc, are the non-key attributes. Note: The same attribute can be of more than one kind. For Example, The Address attribute is a composite attribute, multivalued attribute, stored attribute and a non-key attribute.

What is descriptive attribute?

The attribute(s) used for describing the relationship is called descriptive attributes, also referred as relationship attributes. They are actually used for storing information about the relationship. ... LAD is called as descriptive attribute/relationship attribute associated with the many-to-many relationship ACCESS.

What is a single valued attribute?

Single Valued attribute: Attributes that can have single value at a particular instance of time are called single valued. A person can't have more than one age value. Therefore, age of a person is a single-values attribute.

What is composite attribute?

Composite Attribute – An attribute composed of many other attribute is called as composite attribute. For example, Address attribute of student Entity type consists of Street, City, State, and Country. In ER diagram, composite attribute is represented by an oval comprising of ovals.

Which of the following is a single valued attribute?

Explanation: Register_number is a single valued attribute.

Which of the following can be multi valued attribute?

A multivalued attribute can have more than one value at a time for an attribute. For ex., the skills of a surgeon is a multivalued attribute since a surgeon can have more than one skill. Another common example is the address field, which can have multiple values like zipcode, street address, state, etc.

Which of the following is a derived attribute?

Derived attribute − Derived attributes are the attributes that do not exist in the physical database, but their values are derived from other attributes present in the database. For example, average_salary in a department should not be saved directly in the database, instead it can be derived.

Which one of the following attribute can be taken as a primary key?

Which one of the following attribute can be taken as a primary key? Explanation: The attributes name, street and department can repeat for some tuples. But the id attribute has to be unique. So it forms a primary key.

What is primary key and example?

A primary key is either an existing table column or a column that is specifically generated by the database according to a defined sequence. For example, students are routinely assigned unique identification (ID) numbers, and all adults receive government-assigned and uniquely-identifiable Social Security numbers.

Does every table need a foreign key?

1 Answer. There is no problem having a table that consists of foreign keys only. ... If you add further columns in the future there will be no problems due to the existing columns all being foreign keys. This situation is very common when there is a many-to-many relationship between entity types.

Can foreign key be null?

Short answer: Yes, it can be NULL or duplicate. I want to explain why a foreign key might need to be null or might need to be unique or not unique. First remember a Foreign key simply requires that the value in that field must exist first in a different table (the parent table). ... Null by definition is not a value.

Is foreign key unique?

A foreign key is a column or group of columns in a relational database table that provides a link between data in two tables. It uniquely identifies a record in the relational database table.

Can foreign key be not unique?

No, foreign keys do not have to be unique. Indeed, a lack of uniqueness is requisite for one-to-many or many-to-many relations.

Does a foreign key have to be unique?

By the SQL standard, a foreign key must reference either the primary key or a unique key of the parent table. If the primary key has multiple columns, the foreign key must have the same number and order of columns. ... Any primary key must be unique and non-null.

Can a foreign key not be a primary key?

A foreign key can refer to either a unique or a primary key of the parent table. If the foreign key refers to a non-primary unique key, you must specify the column names of the key explicitly.

Is foreign key can be primary key?

Foreign keys are almost always “Allow Duplicates,” which would make them unsuitable as Primary Keys. It is perfectly fine to use a foreign key as the primary key if the table is connected by a one-to-one relationship, not a one-to-many relationship.

Is unique key same as primary key?

Primary Key is a column that is used to uniquely identify each tuple of the table. It is used to add integrity constraints to the table. Only one primary key is allowed to be used in a table. ... Unique key is a constraint that is used to uniquely identify a tuple in a table.

Is a primary key unique?

The PRIMARY KEY constraint uniquely identifies each record in a table. Primary keys must contain UNIQUE values, and cannot contain NULL values. A table can have only ONE primary key; and in the table, this primary key can consist of single or multiple columns (fields).

What is unique key example?

Example of Creating Unique Key Consider the same student table with attributes, 1) StudID, 2) Roll No, 3) First Name, 4) Last Name, and 5) Email. Stud ID can have a unique constraint where entries in Stud ID column can be unique because each student of a university must have a unique ID number.

What is difference between primary key and index?

A table can have only primary key whereas there can be multiple unique key on a table. A Clustered index automatically created when a primary key is defined whereas Unique key generates the non-clustered index....Comparison Chart:

Paramenter	PRIMARY KEY	UNIQUE KEY
Index	Creates clustered index	Creates non-clustered index

Is primary key an index?

There are four main kinds of indexes; primary keys (unique and not null), unique indexes (unique and can be null), plain indexes (not necessarily unique) and full-text indexes (for full-text searching). The terms 'KEY' and 'INDEX' are generally used interchangeably, and statements should work with either keyword.

Which is faster primary key or index?

If there are many rows covered by the index than it's can be faster to simply do a table scan instead. An index adds some overhead while querying so if the between covers more than 80% (completely arbitrary, but you get the idea) of the rows in the table, the table scan can be faster.

What is foreign key SQL?

The FOREIGN KEY constraint is used to prevent actions that would destroy links between tables. A FOREIGN KEY is a field (or collection of fields) in one table, that refers to the PRIMARY KEY in another table.