Introduction to SQL

Database Concept

DBMS ER Model

DBMS Relational model

DBMS Normalization

DBMS Basic SQL

DBMS DML Command

DBMS DCL Command

Advanced SQL

SELECT query is used to retrieve data from a table. It is the most used SQL query. We can retrieve complete table data, or partial by specifying conditions using the WHERE clause.

Syntax of `SELECT` query

SELECT query is used to retieve records from a table. We can specify the names of the columns which we want in the resultset.

SELECT 
    column_name1, 
    column_name2, 
    column_name3, 
    ...
    column_nameN 
    FROM table_name;

Time for an Example

Consider the following student table,

s_id	name	age	address
101	Adam	15	Chennai
102	Alex	18	Delhi
103	Abhi	17	Banglore
104	Ankit	22	Mumbai

SELECT s_id, name, age FROM student;

The above query will fetch information of s_id, name and age columns of the student table and display them,

s_id	name	age
101	Adam	15
102	Alex	18
103	Abhi	17
104	Ankit	22

As you can see the data from address column is absent, because we did not specif it in our SELECT query.

Select all records from a table

A special character asterisk * is used to address all the data(belonging to all columns) in a query. SELECT statement uses * character to retrieve all records from a table, for all the columns.

SELECT * FROM student;

The above query will show all the records of student table, that means it will show complete dataset of the table.

s_id	name	age	address
101	Adam	15	Chennai
102	Alex	18	Delhi
103	Abhi	17	Banglore
104	Ankit	22	Mumbai

Select a particular record based on a condition

We can use the WHERE clause to set a condition,

SELECT * FROM student WHERE name = 'Abhi';

The above query will return the following result,

103

Abhi

Rohtak

Performing Simple Calculations using `SELECT` Query

Consider the following employee table.

eid	name	age	salary
101	Adam	26	5000
102	Ricky	42	8000
103	Abhi	25	10000
104	Rohan	22	5000

Here is our SELECT query,

SELECT eid, name, salary+3000  FROM employee;

The above command will display a new column in the result, with 3000 added into existing salaries of the employees.

eid	name	salary+3000
101	Adam	8000
102	Ricky	11000
103	Abhi	13000
104	Rohan	8000

So you can also perform simple mathematical operations on the data too using the SELECT query to fetch data from table.

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Database Concept

DBMS ER Model

DBMS Relational model

DBMS Normalization

DBMS Basic SQL

DBMS DML Command

DBMS DCL Command

Advanced SQL

Structure Query Language(SQL) is a database query language used for storing and managing data in Relational DBMS. SQL was the first commercial language introduced for E.F Codd’s Relational model of database. Today almost all RDBMS(MySql, Oracle, Infomix, Sybase, MS Access) use SQL as the standard database query language. SQL is used to perform all types of data operations in RDBMS.

SQL Command

SQL defines following ways to manipulate data stored in an RDBMS.

DDL: Data Definition Language

This includes changes to the structure of the table like creation of table, altering table, deleting a table etc.

All DDL commands are auto-committed. That means it saves all the changes permanently in the database.

Command	Description
create	to create new table or database
alter	for alteration
truncate	delete data from table
drop	to drop a table
rename	to rename a table

DML: Data Manipulation Language

DML commands are used for manipulating the data stored in the table and not the table itself.

DML commands are not auto-committed. It means changes are not permanent to database, they can be rolled back.

Command	Description
insert	to insert a new row
update	to update existing row
delete	to delete a row
merge	merging two rows or two tables

TCL: Transaction Control Language

These commands are to keep a check on other commands and their affect on the database. These commands can annul changes made by other commands by rolling the data back to its original state. It can also make any temporary change permanent.

Command	Description
commit	to permanently save
rollback	to undo change
savepoint	to save temporarily

DCL: Data Control Language

Data control language are the commands to grant and take back authority from any database user.

Command	Description
grant	grant permission of right
revoke	take back permission.

DQL: Data Query Language

Data query language is used to fetch data from tables based on conditions that we can easily apply.

Command	Description
select	retrieve records from one or more table

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Tag: Introduction to SQL

`SELECT` SQL Query

Database Concept

DBMS ER Model

DBMS Relational model

DBMS Normalization

DBMS Basic SQL

DBMS DML Command

DBMS DCL Command

Advanced SQL

Syntax of `SELECT` query

Time for an Example

Select all records from a table

Select a particular record based on a condition

Performing Simple Calculations using `SELECT` Query