(Aktu Btech) Database Management System Important Unit-2 Relational Data Model and Language

With Aktu’s Quantum Notes for Database Management System, you can improve your B.Tech studies. With the help of these crucial, repeating questions and notes, you may master vital topics and do well on tests. Unit-2 Relational Data Model and Language

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Q1. What is relational model ? Explain with example.

Ans.

• 1. A relational model is a collection of conceptual tools for describing data, data relationships, data semantics and consistency constraints.  
• 2. It is the primary data model for commercial data processing applications. 
• 3. The relational model uses collection of tables to represent both data and the relationships among those data. 
• 4. Each table has multiple columns and each column has a unique name.

For example: 

• 1. The tables represent a simple relational database. 
• 2. The Table 2.1.1 shows details of bank customers, Table 2.1.2 shows accounts and Table 2.1.3 shows which accounts belong to which customer.

• 3. The Table 2.1.1, i.e., customer table, shows the customer identified by cust id C_101 is named Ajay and lives in Delhi. 
• 4. The Table 2.1.2, i.e., accounts, shows that account A-1 has a balance of 1000. 
• 5. The Table 2.1.3, i.e., depositor table, shows that account number (ac._no) A-1 belongs to the cust whose cust_id is C_101 and account number (acc_no) A-2 belongs to the cust whose cust_id is C_102 and likewise.

Q2. Explain integrity constraints.  

Ans.

• 1. Integrity constraints provide a way of ensuring that changes made to the database by authorized users do not result in a loss of data consistency. 
• 2. A form of integrity constraint with ER models is:
  • a. key declarations: certain attributes form a candidate key for the entity set.
  • b. form of a relationship: mapping cardinalities 1-1, 1-many and many-many. 
• 3. An integrity constraint can be any arbitrary predicate applied to the database. 
• 4. Integrity constraints are used to ensure accuracy and consistency of data in a relational database. 

Q3. What is relational algebra? Discuss its basic operations. 

Ans.

• 1. The relational algebra is a procedural query language. 
• 2. It consists of a set of operations that take one or two relations as input and produces a new relation as a result. 
• 3. The operations in the relational algebra are select, project, union, set difference, cartesian product and rename.  

Basic relational algebra operations are as follows: 

1. Select operation: 

• a. The select operation selects tuples that satisfies a given predicate. 
• b. Select operation is denoted by sigma (σ). 
• c. The predicate appears as a subscript to σ. 
• d. The argument relation is in parenthesis after the σ. 

2. Project operation: 

• a. The project operation is a unary operation that returns its argument relation with certain attributes left out.
• b. In project operation duplicate rows are eliminated. 
• c. Projection is denoted by pi (ᴨ).

3. Set difference operation: 

• a. The set difference operation denoted by (-) allows us to find tuples that are in one relation but are not in another. 
• b. The expressionr-s produces a relation containing those tuples in r but not in s.

4. Cartesian product operation: 

• a. The cartesian product operation, denoted by a cross (x), allows us to combine information from any two relations. The cartesian product of relations r₁ and r₂ is written as r₁ x r₂.

5. Rename operation: 

• a. The rename operator is denoted by rho (ρ). 
• b. Given a relational algebra expression E, 

returns the result of expression E under the name x. 

• c. The rename operation can be used to rename a relation r to get the same relation under a new name. 
• d. The rename operation can be used to obtain a new relation with new names given to the original attributes of original relation as

Q4. Write short note on SQL. Explain various characteristics of SQL.

Ans.

• 1. SQL stands for Structured Query Language.
• 2. It is a non-procedural language that can be used for retrieval and management of data stored in relational database. 
• 3. It can be used for defining the structure of data, modifying data in the database and specifying the security constraints. 
• 4. The two major categories of SQL commands are:
  • a. Data Definition Language (DDL): DDL provides commands that can be used to create, modify and delete database objects. 
  • b. Data Manipulation Language (DML): DML provides commands that can be used to access and manipulate the data, that is, to retrieve, insert, delete and update data in a database.  

Characteristics of SQL: 

• 1. SQL usage is extremely flexible. 
• 2. It uses a free form syntax that gives the user the ability to structure sQL statements in a way best suited to them. 
• 3. Each SQL request is parsed by the RDBMS before execution, to check for proper syntax and to optimize the request. 
• 4. Unlike certain programming languages, there is no need to start SQL statements in a particular column or be finished in a single line. The same SQL request can be written in a variety of ways.

Q5. Give the brief explanation of view.

Ans.

• 1. A view is a virtual relation, whose contents are derived from already existing relations and it does not exist in physical form. 
• 2. The contents of view are determined by executing a query based on any relation and it does not form the part of database schema. 
• 3. Each time a view is referred to, its contents are derived from the relations on which it is based. 
• 4. A view can be used like any other relation that is, it can be queried, inserted into, deleted from and joined with other relations or views. 
• 5. Views can be based on more than one relation and such views are known as complex views.
• 6. A view in SQL terminology is a single table that is derived from other tables. These other tables can be base tables or previously defined views.  

Syntax for creating view: 

Q6. Explain embedded SQL and dynamic SQL in detail.

Ans. Embedded SQL: 

• 1. The SQL standard defines embeddings of SQL in a variety of programming languages such as Pascal, PL/I, Fortran, C and COBOL. 
• 2. A language in which SQLqueries are embedded is referred to as a host language and the SQL structures permitted in the host language constitute embedded SQL. 
• 3. Programs written in the host language can use the embedded SQL syntax to access and update data stored in a database. 
• 4. In embedded SQL, all query processing is performed by the database system. 
• 5. The result of the query is then made available to the program one tuple at a time. 
• 6. Embedded SQL statements must be completely present at compile time and compiled by the embedded SQL preprocessor.
• 7. To identify embedded SQL requests to the preprocessor, we use the EXEC, SQL statement as: 

• 8. Variable of the host language can be used within embedded SQL statements, but they must be preceded by a colon (:) to distinguish them from SQL variables.

Dynamic SQL: 

• 1. The dynamic SQL component of SQL allows programs to construct and submit SQL queries at run time. 
• 2. Using dynamic SQL, programs can create SQL queries as strings at run time and can either have them executed immediately or have them prepared for subsequent use. 
• 3. Preparing a dynamic SQL statement compiles it, and subsequent uses of the prepared statement use the compiled version.
• 4. SQL defines standards for embedding dynamic SQL calls in a host language, such as C, as in the following example,

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