What are the main reasons for introducing the concepts of superclasses and subclasses into an ER model?

What are the main reasons for introducing the concepts of superclasses and subclasses into an ER model?

Two reasons for introducing the concepts of superclasses and subclasses into an ER model: It avoids describing similar concepts more than once, there by saving time for the designer and making the ER diagram more readable. It adds more semantic information to the design in a form that is familiar to many people.

Why do we use specialization and generalization in ER?

Generalization, Specialization and Aggregation in ER model are used for data abstraction in which abstraction mechanism is used to hide details of a set of objects. Generalization is the process of extracting common properties from a set of entities and create a generalized entity from it.

What is generalization and specialization in database?

Specialization and generalization are main principles of database modeling. Specialization is based on a refinement of types or classes to more specific ones. Generalization maps or groups types or classes to more abstract or combined ones.

What is ER model explain its concepts?

ER model stands for an Entity-Relationship model. It is a high-level data model. This model is used to define the data elements and relationship for a specified system. It develops a conceptual design for the database. It also develops a very simple and easy to design view of data.

What is key in ER diagram?

A key is an attribute in ER diagrams whose values are distinct for each individual entity in an entity set. For example for a student entity set Roll Number or Registration Number can be a key because these are unique for each student entity.

How do we indicate primary key in ER diagram?

ER Diagram: Key Attribute Key attribute represents the main characterstic of an Entity. It is used to represent a Primary key. Ellipse with the text underlined, represents Key Attribute.

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 relationships in an ER diagram have a primary key?

Validate Keys and Relationships Every entity in the data model shall have a primary key whose values uniquely identify entity instances. The primary key attribute cannot be optional (i.e., have null values). The primary key cannot have repeating values.

Which 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.

Can relationships have attributes in an ER diagram?

In ER model, entities have attributes which can be of various types like single-valued, multi-valued, composite, simple, stored, derived and complex. But relationships can also have attributes associated to them.

What is PK in ERD?

A primary key (PK) is one or more data attributes that uniquely identify an entity. A key that consists of two or more attributes is called a composite key.

What does PK and FK mean?

Primary key (PK) – value which uniquely identifies every row in the table. Foreign keys (FK) – values match a primary or alternate key inherited from some other table. Primary Foreign Key (PFK) – key which acts as primary key in more than one table which occurs in tables with an identifying relationship.

What is a PK column?

In the relational model of databases, a primary key is a specific choice of a minimal set of attributes (columns) that uniquely specify a tuple (row) in a relation (table). Informally, a primary key is “which attributes identify a record”, and in simple cases are simply a single attribute: a unique id.

Why primary key is used in SQL?

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 the difference between primary key and composite key?

Primary key is that column of the table whose every row data is uniquely identified. Composite Key is a form of the candidate key where a set of columns will uniquely identify every row in the table.

What is a single column primary key?

To identify a primary key for a table or file, you locate values of a column that uniquely identify the rows in the selected table. Additionally, you can verify that a primary key that is defined in the data source is the true primary key.

How do you make a column primary key?

To create a primary key

1. In Object Explorer, right-click the table to which you want to add a unique constraint, and click Design.
2. In Table Designer, click the row selector for the database column you want to define as the primary key.
3. Right-click the row selector for the column and select Set Primary Key.

How do you check if a column is a primary key in SQL?

Here are a few lines of sql query using which we can get the primary column name.

1. select C.COLUMN_NAME FROM.
2. INFORMATION_SCHEMA.TABLE_CONSTRAINTS T.
3. JOIN INFORMATION_SCHEMA.CONSTRAINT_COLUMN_USAGE C.
4. ON C.CONSTRAINT_NAME=T.CONSTRAINT_NAME.
5. WHERE.
6. C.TABLE_NAME=’Employee’
7. and T.CONSTRAINT_TYPE=’PRIMARY KEY’

How do you change the primary key of a table in SQL?

To modify a primary key, execute an ALTER TABLE… DROP PRIMARY KEY statement to drop the existing primary key, and then execute an ALTER TABLE… ALTER column-name column-alteration statement to set the new primary key for the table.

How do I change the primary key of a data type?

2 Answers. You cannot change datatypes of Key fields (Primary or Foreign). You can’t even change a datatype if the field is used in an index.

Can we update primary key in mysql?

3 Answers. Next time, use a single “alter table” statement to update the primary key. alter table xx drop primary key, add primary key(k1, k2, k3);

How many primary keys can have in a table?

one primary key

Can we have 2 primary keys in a table in mysql?

You can only have one primary key, but you can have multiple columns in your primary key. You can also have Unique Indexes on your table, which will work a bit like a primary key in that they will enforce unique values, and will speed up querying of those values.

How many primary keys can a table have Mcq?