Computer Science: DBMS

Contents

• Introduction: RDBMS, Table/relation, tupple, attribute
• Key in RDBMS
• Structure Query Language: DDL, DCL, DML, Transitional language..
• Example of various queries. 

Relational Database
A relational database is a collection of data items organized as logically related tables.
The software required to handle/manipulate these table/relations is know as Relational Database Management System (RDBMS). Example -
Oracle, Sybase, DB2, MSSQL, etc.

Table/Relation

A group of rows and columns form a table. 

The horizontal subset of the Table is known as a Row/Tuple. 

  - Cardinality: No. of Rows of Table

The vertical subset of the Table is known as a Column/an Attribute. 

  - Degree: No.of columns of Table.

Key in RDBMS
To maintain data integrity (that is data should be correct and in well formed) we use concept of keys. There are five types of keys in database which is as follows -

1. Candidate key
2. Primary Key
3. Foreign Key
4. Alternate Key
5. Composite Key

Candidate Key
Candidate keys are those keys which is candidate for primary key of a table. In simple words we can understand that such type of keys which full fill all the requirements of primary key which is not null and have unique records is a candidate for primary key. So thus type of key is known as candidate key. Every table must have at least one candidate key but at the same time can have several.

Primary Key
Such type of candidate key which is chosen as a primary key for table is known as primary key. Primary keys are used to identify tables. There is only one primary key per table. In SQL Server when we create primary key to any table then a clustered index is automatically created to that column.

Foreign Key
Foreign key are those keys which is used to define relationship between two tables. When we want to implement relationship between two tables then we use concept of foreign key. It is also known as referential integrity. We can create more than one foreign key per table.  foreign key is generally a primary key from one table that appears as a field in another where the first table has a relationship to the second. In other words, if we had a table A with a primary key X that linked to a table B where X was a field in B, then X would be a foreign key in B.

Alternate Key
If any table have more than one candidate key, then after choosing primary key from those candidate key, rest of candidate keys are known as an alternate key of  that table. Like here we can take a very simple example to understand the concept of alternate key. Suppose we have a table named Employee which has two columns EmpID and EmpMail, both have not null attributes and unique value. So both columns are treated as candidate key. Now we make EmpID as a primary key to that table then EmpMail is known as alternate key.

Composite Key
When we create keys on more than one column then that key is known as composite key. Like here we can take an example to understand this feature. I have a table Student which has two columns Sid and SrefNo and we make primary key on these two column. Then this key is known as composite key.

SQL- Structured Query Language
SQL commands classified by function:

1. Data definition language (DDL) - used to define or change database structure(s) (e.g., CREATE, ALTER, DROP)
2. Data manipulation language (DML) - used to select or change data (e.g., INSERT, UPDATE, DELETE, SELECT)
3. Data control language (DCL) - used to control user access (e.g., GRANT, REVOKE)
4. Transnational language - used to control logical units of work (e.g., COMMIT,)

Creating a new table in the database
Syntax :

CREATE TABLE table_name
(
column_name1 data_type(size),
column_name2 data_type(size),
column_name3 data_type(size),
....
);

Example :

CREATE TABLE student
(
    rno int, 
    name char(25), 
    fees int, 
    dob date,
    class char(3)
); 

Inserting a new row at the bottom of the table
Syntax :

INSERT INTO table_name
VALUES (value1,value2,value3,...);

You can also specifies both the column names and the values to be inserted:

INSERT INTO table_name (column1,column2,column3,...)
VALUES (value1,value2,value3,...);

Examples

INSERT INTO student 
VALUES(10, 'Alex', 7800, '1998-10-03','K12');
INSERT INTO student(rno, name, fees, dob, class) 

values(11, 'Peter', 6700, '1997-11-15', 'K12');



Displaying the content from a table – SELECT
Example : SELECT * FROM student;

rno	name	fees	dob	class
10	Alex	7800	1998-10-03	K12
11	Peter	6700	1997-11-15	K12
12	Alisha	7800	1999-07-03	K11
13	John	6900	2000-12-13	K11

SELECT name FROM student;

name
Alex
Peter
Alisha
John

Relational Operator
=, <, >, <=, >=, <>

Logical Operator

AND, OR, NOT

SELECT * FROM student WHERE fees < 7000;

rno	name	fees	dob	class
11	Peter	6700	1997-11-15	K12
13	John	6900	2000-12-13	K11

SELECT * FROM student WHERE fess > 7000 AND fees < 8000;

rno	name	fees	dob	class
10	Alex	7800	1998-10-03	K12
12	Alisha	7800	1999-07-03	K11

SELECT * FROM student WHERE fees > 7000 OR class = 'K12';

rno	name	fees	dob	class
10	Alex	7800	1998-10-03	K12
11	Peter	6700	1997-11-15	K12
12	Alisha	7800	1999-07-03	K11

SELECT name, fees FROM student WHERE NOT (class = 'K12');

name	fees
Alisha	7800
John	6900

SELECT name, fees FROM student WHERE class <> 'K12';

note: here <> is denote not equal.

name	fees
Alisha	7800
John	6900

SELECT * FROM student WHERE rno IN(10, 12, 13);

rno	name	fees	dob	class
10	Alex	7800	1998-10-03	K12
12	Alisha	7800	1999-07-03	K11
13	John	6900	2000-12-13	K11

SELECT * FROM student WHERE rno BETWEEN 11 AND 13;

rno	name	fees	dob	class
11	Peter	6700	1997-11-15	K12
12	Alisha	7800	1999-07-03	K11
13	John	6900	2000-12-13	K11

SELECT name FROM student WHERE name LIKE 'A%';

name

Alex

Alisha

SELECT * name FROM student WHERE name LIKE '%a';

rno	name	fees	dob	class
12	Alisha	7800	1999-07-03	K11

SELECT name FROM student WHERE Name LIKE '%e%' ;

name

Alex

Peter

Modifying the existing content of the table

Syntax:

UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;

Example

UPDATE student
SET fees = '7900'
WHERE rno = 12 ;

SELECT * FROM student;  

rno	name	fees	dob	class
10	Alex	7800	1998-10-03	K12
11	Peter	6700	1997-11-15	K12
12	Alisha	7900	1999-07-03	K11
13	John	6900	2000-12-13	K11

Arranging the data in ascending or descending order of one/multiple columns (ORDER BY clause)

Syntax:

SELECT column_name,column_name
FROM table_name
ORDER BY column_name,column_name ASC|DESC;

Example

SELECT * FROM student ORDER BY name;

rno	name	fees	dob	class
10	Alex	7800	1998-10-03	K12
12	Alisha	7900	1999-07-03	K11
13	John	6900	2000-12-13	K11
11	Peter	6700	1997-11-15	K12

SELECT * FROM student ORDER BY fees DESC;

rno	name	fees	dob	class
12	Alisha	7900	1999-07-03	K11
10	Alex	7800	1998-10-03	K12
13	John	6900	2000-12-13	K11
11	Peter	6700	1997-11-15	K12

SELECT class, name, dob, fees FROM student ORDER BY class, name DESC;

class	name	dob	fees
K11	John	2000-12-13	6900
K11	Alisha	1999-07-03	7900
K12	Peter	1997-11-15	6700
K12	Alex	1998-10-03	7800

SELECT class, name, fees, fees*12 annualfees FROM student;

class	name	fees	annualfees
K12	Alex	7800	93600
K12	Peter	6700	80400
K11	Alisha	7900	94800
K11	John	6900	82800

Using Aggregate Functions with SELECT

COUNT( ) To count the number of rows

SUM( ) To find the sum of values in the column

MAX( ) To find the maximum value in the column

MIN( ) To find the minimum value in the column

AVG( ) To find the average of values in the column

SELECT COUNT(*) FROM student;

COUNT(*)

4

SELECT COUNT(rno) FROM student;

COUNT(rno)

4

SELECT SUM(fees) FROM student;

SUM(fees)

29300

SELECT AVG(fees) FROM student;

AVG(fees)

7325.0000

SELECT MAX(fees), MIN(fees) FROM student;

MAX(fees)	MIN(fees)
7900	6700

Grouping data under given Column- (GROUP BY)

SELECT class, SUM(fees) FROM student GROUP BY class;

class	SUM(fees)
K11	14800
K12	14500

SELECT class, MAX(fees), MIN(fees) FROM student GROUP BY class;

class	MAX(fees)	MIN(fees)
K11	7900	6900
K12	7800	6700

SELECT class, MAX(dob) FROM student GROUP BY class HAVING COUNT(*)>1; 

class	MAX(dob)
K11	2000-12-13
K12	1998-10-03

 

Deleting a row/rows from a table

Syntax:

DELETE FROM table_name
WHERE some_column=some_value;

Example:

DELETE FROM Student WHERE rno = 13;

Adding a new column(s) in the table

Syntax :

ALTER TABLE table_name
ADD column_name datatype

Examples :

ALTER TABLE student ADD (grade CHAR(2));

 

Modifying the data type of a column

Syntax;

ALTER TABLE table_name
MODIFY column_name datatype

Example:

ALTER TABLE student MODIFY (grade CHAR(1));

 

Deleting a table

Syntax:

DROP TABLE table_name

Example:

DROP TABLE student;

 

Working with more than one table

Syntax:

SELECT col1, col2, col3...
FROM table_name1, table_name2 
WHERE table_name1.col2 = table_name2.col1;

Table - product

product_id	product_name	supplier_name	unit_price
100	Camera	Nikon	300
101	Television	Onida	100
102	Refrigerator	Videocon	150
103	Ipod	Apple	75
104	Mobile	Nokia	50

Table - order_items

order_id	product_id	total_units	customer
5100	104	30	Infosys
5101	102	5	Satyam
5102	103	25	Wipro
5103	101	10	TCS

SELECT order_id, product_name, unit_price, supplier_name, total_units 
FROM product, order_items 
WHERE order_items.product_id = product.product_id;