Which of the following is an example for 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.
Can two entities have the same attributes?
1 Answer. It is ok to have tables with the same attributes. Just find sufficient meanings/predicates to record your application state.
Can a relationship have an attribute?
Like entities, relationships can have attributes: we can define a sale to be a relationship between a customer entity (identified by the unique email address) and a given number of the product entity (identified by the unique product ID) that exists at a particular date and time (the timestamp).
What do attributes represent in an ER model?
Attributes are the properties of entities. Attributes are represented by means of ellipses. Every ellipse represents one attribute and is directly connected to its entity (rectangle). If the attributes are composite, they are further divided in a tree like structure.
What is a relationship attribute?
Attribute relationships are associations between attributes that specify how attributes are connected. Attribute relationships define how tables and columns are joined and used, and which tables are related to other tables. Without relationships, there is no interaction between data, and therefore no logical structure.
What is the key attribute?
A key attribute is the unique characteristic of the entity. For ex. Name and hire date are attributes of the entity Employee.
Can a relationship have a primary key?
Columns that define primary keys in one table in a relational model can have a relationship with columns in one or more other tables. Every table can have (but does not have to have) a primary key. The column or columns defined as the primary key ensure uniqueness in the table; no two rows can have the same key.
Can 2 tables have same primary key?
Yes. You can have same column name as primary key in multiple tables. Column names should be unique within a table. A table can have only one primary key, as it defines the Entity integrity.
Can there be no primary key?
The short answer is: NO. The purpose of the primary key is to uniquely identify a row on the table in order to form a relationship with another table. Traditionally, an auto-incremented integer value is used for this purpose, but there are variations to this.
Is primary key mandatory?
A “primary key” in MS SQL consists of a unique index (clustered by default) and a constraint to enforce uniqueness of the field. If you are not inserting into the table on the reporting database, there is no reason for the constraint. There is no requirement the index be clustered.
What’s the difference between a primary key and a unique 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 always unique?
3 Answers. Primary key is always unique in every SQL. You dont have to explicitly define it as UNIQUE. Also you can have only one primary key constraint in the table(as the point of creating a primary key is to uniquely identify the row in your table) but you can more than one unique key constraint in your table.
Which is a unique key?
A unique key is a group of one or more than one fields or columns of a table which uniquely identify database record. A unique key is the same as a primary key, but it can accept one null value for a table column. It also cannot contain identical values.
Is foreign key unique?
S.NO. A primary key is used to ensure data in the specific column is 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?
BUT, as it is the Primary Key of another table, it must be unique in this table. No. But the values must exists first on the parent table before you can insert it on the table. No, foreign keys do not have to be unique.
Can a table not have 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.
What are the requirements for a foreign key field?
- The foreign key must be a primary key on at least one of the tables.
- The field types for the foreign key field must be the same on both tables.
- The info being related must be the same.
Can foreign key have multiple values?
A table may have multiple foreign keys, and each foreign key can have a different parent table. Each foreign key is enforced independently by the database system. Therefore, cascading relationships between tables can be established using foreign keys.
What is alternate key?
Alternate Key or Secondary Key is the key that has not been selected to be the primary key, but are candidate keys. However, it is considered a candidate key for the primary key. A candidate key not selected as a primary key is called alternate or secondary key.
What is a primary key field?
A primary key is a field or set of fields with values that are unique throughout a table. Values of the key can be used to refer to entire records, because each record has a different value for the key. Each table can only have one primary key.