![Explain the GROUP BY and HAVING clause
in SQL?
GROUP BY
GROUP BY is an optional clause in a query. It follows
WHERE clause or the FROM clause if the WHERE clause
is missing.
GROUP BY has the following general format:
GROUP BY column-1 [column-2]
Each group we consist of tuples that have the same
value of attribute called grouping attribute.](https://image.slidesharecdn.com/previousquestionpapers-150405210323-conversion-gate01/85/Previous-question-papers-of-Database-Management-System-DBMS-By-SHABEEB-14-320.jpg)
Previous question papers of Database Management System (DBMS) By SHABEEB
- 1. Previous Question Papers Presented By: Mohammed Shabeeb KV No:233
- 2. Data model tells how the logical structure of a database is modeled. Data Models are fundamental entities to introduce abstraction in DBMS. Data models define how data is connected to each other and how it will be processed and stored inside the system. Hierarchical Model Network Model Relational Model Explain about various data models used to describe the design of a database?
- 3. Hierarchical Model The hierarchical data model organizes data in a tree structure. One to many relationship. The structure is based on the rule that one parent can have many children but children are allowed only one parent.
- 4. Network Model Some data were more naturally modeled with more than one parent per child. So, the network model permitted the modelling of many-to-many relationships in data.
- 5. Relational Model It defines a database as a collection of tables (relations) which contains all data. Each table has multiple columns, and each column has a unique name. Properties of Relational Tables: Values are atomic. Each row is unique. Column values are of the same kind. The sequence of columns is insignificant. The sequence of rows is insignificant. Each column has a unique name.
- 6. Explain about the constraints in SQL? SQL constraints are used to specify rules for the data in table. These constraints ensure database integrity and hence they are often called database integrity constraints. The following are the constraints: NOT NULL Column must always have a value. It can never be null.
- 7. UNIQUE No two rows can have the same value for the UNIQUE KEY column or columns. PRIMARY KEY No two rows in the table can have the same value. Primary key cannot contain a NULL value this constraint must be defined only on the column defined as NOT NULL. DEFAULT Using this constraint, a default value can be set for a column, when the user does not enter a value for the column. CHECK This constraint limits value that can be inserted into a column of a table.
- 8. What is the difference between database schema and a database instance? The overall design of a data base is called a schema. The collection of information stored in the database at a particular moment is called instance of a database or database state. A database schema is corresponds to the variable declaration in a program. Each variable has a particular value at a given instant. The value of variable in a program at a point in time correspond to an instance of a database schema.
- 9. Explain about view and its advantages? Any relation that is not part of the logical model, but is made visible to a user as a virtual relation is called view. A view is a relation that is derived from other relation . These other relation could be a base relation or previously defined view. Advantages: View mechanism is that it provide the support for the logical data independence. View are also valuable in the context of security. Data integrity. Shielding from change. Easier querying.
- 10. Distinguish between primary key, composite key and candidate key? PRIMARY KEY It is a candidate key that is used by the data base designers for unique identification of each row in a table. It can be consist of one or more attribute. Eg:- ID key is primary key.
- 11. CANDIDATE KEY It is defined as minimal super key. It is a combination of attribute that identifies the record uniquely. But no one of it is proper subsets can identify the records uniquely. Eg: 1.ID 2.Name,address
- 12. COMPOSITE KEY If we use multiple attribute to create a primary key then that primary key is called composite key. Also called compound key or concatenated key. Eg:-If we have used second name , address as a primary key then it will be our composite key.
- 13. What is meant by normalization? Normalization is the process of decomposing unsatisfactory bad relations by breaking up their attributes to small relations. NORMAL FORM Condition using key & functional dependencies of a relation to certify whether a relation schema is in a particular normal form.
- 14. Explain the GROUP BY and HAVING clause in SQL? GROUP BY GROUP BY is an optional clause in a query. It follows WHERE clause or the FROM clause if the WHERE clause is missing. GROUP BY has the following general format: GROUP BY column-1 [column-2] Each group we consist of tuples that have the same value of attribute called grouping attribute.
- 15. HAVING The HAVING clause is associated with grouping queries and aggregate queries. It is optional in both case. In grouping queries it follows the GROUP BY clause . In aggregate queries HAVING clause follows the WHERE clause or the FROM clause if the WHERE clause missing. The HAVING clause has the following general format: HAVING predicate
- 16. What are the purpose of database system? Database management systems were developed to handle the following difficulties of typical file-processing systems supported by conventional operating systems: Data redundancy and inconsistency Difficulty in accessing data Data isolation – multiple files and formats Integrity problems Atomicity of updates Concurrent access by multiple users Security problems
- 17. Thank You