1. Introduction to Structured Query Language(SQL)
Prepared by Naveedullah Safi

2. What is SQL?
SQL is Structured Query Language, which is a computer language for storing, manipulating and retrieving data stored in relational database.
SQL is the standard language for Relation Database System. All relational database management systems like MySQL, MS Access, Oracle, Sybase, Informix, postgres and SQL Server use SQL as standard database language.

3. Four Main Categories Of SQL Statements

4. SQL Commands:
The standard SQL commands to interact with relational databases are CREATE, SELECT, INSERT, UPDATE, DELETE and DROP. These commands can be classified into groups based on their nature:

5. SQL Commands: DML- Data Manipulation Language:
Description
SELECT
Retrieves certain records from one or more tables
INSERT
Creates a record
UPDATE
Modifies existing records
DELETE
Deletes records
DDL - Data Definition Language:
Command
Description
CREATE
Creates a new table, a view of a table, or other object in database
ALTER
Modifies an existing database object, such as a table.
DROP
Deletes an entire table, a view of a table or other object in the database.

6. SQL Syntax
SQL is followed by unique set of rules and guidelines called Syntax. This tutorial gives you a quick start with SQL by listing all the basic SQL Syntax:
All the SQL statements start with any of the keywords like SELECT, INSERT, UPDATE, DELETE, ALTER, DROP, CREATE, USE, SHOW and all the statements end with a semicolon (;).
Important point to be noted is that SQL is not case sensitive, which means SELECT and select have same meaning in SQL statements.

7. Data manipulation language
DML is abbreviation of Data Manipulation Language. It is used to retrieve, store, modify, delete, insert and update data in database
DML Examples
SELECT – retrieve data from the a database.
INSERT – insert data into a table.
UPDATE – updates existing data within a table.
DELETE – Delete all records from a database table.

8. SQL SELECT Statement:
The SELECT statement is used to select data from a database.
The result is stored in a result table, called the result- set.
SELECT column1, column2....columnN FROM table_name;

9. SELECT Statement Example
The following SQL statement selects the "CustomerName" and "City" columns from the "Customers" table:
Example
SELECT CustomerName, City FROM Customers;
SELECT * Example
The following SQL statement selects all the columns from the "Customers" table:
SELECT * FROM Customers;

10. INSERT INTO Statement
The INSERT INTO statement is used to insert new records in a table.
It is possible to write the INSERT INTO statement in two forms.
The first form does not specify the column names where the data will be inserted, only their values:
The second form specifies both the column names and the values to be inserted:
INSERT INTO table_name VALUES (value1,value2,value3,...);
INSERT INTO table_name (column1,column2,column3,...) VALUES (value1,value2,value3,...);

11. INSERT INTO Example
The following SQL statement inserts a new record in the "Customers" table:
Example
INSERT INTO Customers (CustomerName, ContactName, Address, City, PostalCode, Country) VALUES ('Cardinal', 'Tom B. Erichsen', 'Skagen 21', 'Stavanger', '4006', 'Norway');

12. UPDATE Statement
The UPDATE statement is used to update existing records in a table.
SQL UPDATE Syntax:
Notice the WHERE clause in the SQL UPDATE statement! The WHERE clause specifies which record or records that should be updated. If you omit the WHERE clause, all records will be updated!
UPDATE table_name SET column1=value1,column2=value2,... WHERE some_column=some_value;

13. UPDATE Example
The following SQL statement updates the first customer (CustomerID = 1) with a new contact person and a new city.
Example
UPDATE Customers SET ContactName = 'Alfred Schmidt', City= 'Frankfurt' WHERE CustomerID = 1;

14. DELETE Statement
The DELETE statement is used to delete rows in a table.
SQL DELETE Syntax:
Notice the WHERE clause in the SQL DELETE statement! The WHERE clause specifies which record or records that should be deleted. If you omit the WHERE clause, all records will be deleted!
It is possible to delete all rows in a table without deleting the table.
DELETE FROM table_name WHERE some_column=some_value;
DELETE FROM table_name; or DELETE * FROM table_name;

15. DELETE Statement Example
The following SQL statement deletes the customer "Alfreds Futterkiste" from the "Customers" table:
Example
DELETE FROM Customers WHERE CustomerName='Alfreds Futterkiste';

16. The SQL WHERE Clause The WHERE clause is used to filter records.
The WHERE clause is used to extract only those records that fulfill a specified condition.
WHERE Syntax
SELECT column1, column2, ... FROM table_name WHERE condition;
Note: The WHERE clause is not only used in SELECT statement, it is also used in UPDATE, DELETE statement, etc.!

17. WHERE Clause Example
The following SQL statement selects all the customers from the country "Mexico", in the "Customers" table:
Example
SELECT * FROM Customers WHERE Country='Mexico';

18. ORDER BY Keyword The ORDER BY keyword is used to sort the result-set.
The ORDER BY keyword is used to sort the result-set by one or more columns.
The ORDER BY keyword sorts the records in ascending order by default. To sort the records in a descending order, you can use the DESC keyword.
SQL ORDER BY Syntax
SELECT column_name, column_name FROM table_name ORDER BY column_name ASC|DESC, column_name ASC|DESC;

19. ORDER BY Examples: SELECT * FROM Customers ORDER BY Country;
The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" column:
SELECT * FROM Customers ORDER BY Country;
The following SQL statement selects all customers from the "Customers" table, sorted DESCENDING by the "Country" column:
SELECT * FROM Customers ORDER BY Country DESC;
The following SQL statement selects all customers from the "Customers" table, sorted by the "Country" and the "CustomerName" column:
SELECT * FROM Customers ORDER BY Country, CustomerName;
The following SQL statement selects all customers from the "Customers" table, sorted ascending by the "Country" and descending by the "CustomerName" column:
SELECT * FROM Customers ORDER BY Country ASC, CustomerName DESC;