SQL for Beginners

SQL - Structure Query Language for Beginners- Database.

We continue our journey into the world of SQL and relational database systems.
First we know, SQL is short for Structured Query Language and is a widely used database language, providing means of data manipulation (store, retrieve, update, delete) and database creation.
Almost all modern Relational Database Management Systems like MS SQL Server, Microsoft Access, MSDE, Oracle, DB2, Sybase, MySQL, Postgres and Informix use SQL as standard database language. Now a word of warning here, although all those RDBMS use SQL, they use different SQL dialects. For example MS SQL Server specific version of the SQL is called T-SQL, Oracle version of SQL is called PL/SQL, MS Access version of SQL is called JET SQL, etc.
The foundation of every Relational Database Management System is a database object called table. Every database consists of one or more tables, which store the database’s data/information. Each table has its own unique name and consists of columns and rows.
It look like this :
Table: Customers

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222
Steven	Goldfish	goldfish@fishhere.net	4/4/1974	323 455-4545
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

Then:
The SQL SELECT statement is used to select data from a SQL database table. This is usually the very first SQL command every SQL newbie learns and this is because the SELECT SQL statement is one of the most used SQL commands.
Please have a look at the general SQL SELECT syntax:
SELECT Column1, Column2, Column3,
FROM Table1
Let say e.g.
SELECT lastname, firstname, middlename FROM name
The list of column names after the SQL SELECT command determines which columns you want to be returned in your result set. If you want to select all columns from a database table, you can use the following SQL statement:
SELECT *
FROM Table1
When the list of columns following the SELECT SQL command is replaced with asterix (*) all table columns are returned. Word of caution here, it’s always better to explicitly specify the columns in the SELECT list, as this will improve your query performance significantly.
The SQL WHERE clause is used to select data conditionally, by adding it to already existing SQL SELECT query. We are going to use the Customers table from the previous chapter, to illustrate the use of the SQL WHERE command.
Table: Customers

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222
Steven	Goldfish	goldfish@fishhere.net	4/4/1974	323 455-4545
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

If we want to select all customers from our database table, having last name 'Smith' we need to use the following SQL syntax:
SELECT *
FROM Customers
WHERE LastName = 'Smith'
The result of the SQL expression above will be the following:

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

SQL UPDATE

The SQL UPDATE general syntax looks like this:
UPDATE Table1
SET Column1 = Value1, Column2 = Value2
WHERE Some_Column = Some_Value
The SQL UPDATE clause changes the data in already existing database row(s) and usually we need to add a conditional SQL WHERE clause to our SQL UPDATE statement in order to specify which row(s) we intend to update.
If we want to update the Mr. Steven Goldfish's date of birth to '5/10/1974' in our Customers database table

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 222-2222
Steven	Goldfish	goldfish@fishhere.net	4/4/1974	323 455-4545
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232
James	Smith	jim@supergig.co.uk	20/10/1980	416 323-8888

we need the following SQL UPDATE statement:
UPDATE Customers
SET DOB = '5/10/1974'
WHERE LastName = 'Goldfish' AND FirstName = 'Steven'
If we don’t specify a WHERE clause in the SQL expression above, all customers' DOB will be updated to '5/10/1974', so be careful with the SQL UPDATE command usage.
We can update several database table rows at once, by using the SQL WHERE clause in our UPDATE statement. For example if we want to change the phone number for all customers with last name Smith (we have 2 in our example Customers table), we need to use the following SQL UPDATE statement:
UPDATE Customers
SET Phone = '626 555-5555'
WHERE LastName = 'Smith'
After the execution of the UPDATE SQL expression above, the Customers table will look as follows:

FirstName	LastName	Email	DOB	Phone
John	Smith	John.Smith@yahoo.com	2/4/1968	626 555-5555
Steven	Goldfish	goldfish@fishhere.net	4/4/1974	323 455-4545
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232
James	Smith	jim@supergig.co.uk	20/10/1980	626 555-5555

SQL DELETE

The SQL DELETE command has the following generic SQL syntax:
DELETE FROM Table1
WHERE Some_Column = Some_Value
If you skip the SQL WHERE clause when executing SQL DELETE expression, then all the data in the specified table will be deleted. The following SQL statement will delete all the data from our Customers table and we’ll end up with completely empty table:
DELETE FROM Table1
If you specify a WHERE clause in your SQL DELETE statement, only the table rows satisfying the WHERE criteria will be deleted:
DELETE FROM Customers
WHERE LastName = 'Smith'
The SQL query above will delete all database rows having LastName 'Smith' and will leave the Customers table in the following state:

FirstName	LastName	Email	DOB	Phone
Steven	Goldfish	goldfish@fishhere.net	4/4/1974	323 455-4545
Paula	Brown	pb@herowndomain.org	5/24/1978	416 323-3232

So far, this discussion is my reference in so many website i had searching i would learn.. So if you read this. You will be learn on how the sql works.
Reference:
By Art Branch Inc.
Email Me: vashalowen@gmail.com