DBMS - SQL Queries with an example

1. Basic Structure of SQL Queries
Mrs.R.SABITHA .,M.Sc.,M.Phil.,
Assistant Professor,
Department of Computer Science(SF)
V.V.Vanniaperumal College for Women,
Virudhunagar.

2.  Overview of the SQL Query Language
 Data Definition
 Basic Query Structure
 Additional Basic Operations
 Set Operations
 Null Values
 Aggregate Functions
 Nested Sub queries

3. Overview of the SQL Query
Language
• SQL is a language to operate databases; it includes
Database Creation, Database Deletion, Fetching Data Rows,
Modifying & Deleting Data rows, etc.
• SQL stands for Structured Query Language which is a
computer language for storing, manipulating and retrieving
data stored in a relational database.

4. SQL Basic Commands
DDL (Data Definition Languages)
1. Create - Creates a new table, a view of a table, or other
object in the database.
2. Alter - Modifies an existing database object, such as a
table.
3. Drop - Deletes an entire table, a view of a table or
other objects in the database.

5. SQL Basic Commands
DML - Data Manipulation Language
1. SELECT - Retrieves certain records from one or more
tables.
2. Update – Modify records
3 .Delete – Delete Records

6. Basic Query Structure
 A typical SQL query has the form:
select A1, A2, ..., An
from r1, r2, ..., rm
where P
 Ai represents an attribute
 Ri represents a relation
 P is a predicate.
 The result of an SQL query is a relation.

7. Select Clause
The select clause lists the attributes desired in the result of a query
corresponds to the projection operation of the relational algebra
Example: find the names of all instructors:
select name from instructor
NOTE: SQL names are case insensitive (i.e., you may use upper- or lower-case
letters.)
E.g., Name ≡ NAME ≡ name

8. Select Clause
SQL allows duplicates in relations as well as in query results.
To force the elimination of duplicates, insert the keyword distinct
after select.
Find the department names of all instructors, and remove duplicates
select distinct dept_name
from instructor
The keyword all specifies that duplicates should not be removed.
select all dept_name
from instructor

9. The from Clause
 The from clause lists the relations involved in the query
 Corresponds to the Cartesian product operation of the relational algebra.
 Find the Cartesian product instructor X teaches
select X
from instructor, teaches
 generates every possible instructor – teaches pair, with all attributes from
both relations.
 For common attributes (e.g., ID), the attributes in the resulting table are
renamed using the relation name (e.g., instructor.ID)

10. The where Clause
 The where clause specifies conditions that the result must satisfy
 Corresponds to the selection predicate of the relational algebra.
 To find all instructors in Comp. Sci. dept
select name
from instructor
where dept_name = ‘Comp. Sci.'
 To find all instructors in Comp. Sci. dept with salary > 80000
select name
from instructor
where dept_name = ‘Comp. Sci.' and salary > 80000

11. Examples
 Find the names of all instructors who have taught some course and the
course_id
 select name, course_id
from instructor , teaches
where instructor.ID = teaches.ID
 Find the names of all instructors in the Art department who have taught
some course and the course_id
 select name, course_id
from instructor , teaches
where instructor.ID = teaches.ID and instructor. dept_name = ‘Art’

12. ThankYou