When I must use database like PostgreSQL , MySql , SQLite MSSql and Oracle etc at web server to Build e-commerce website ?

as we need to enter some data bases

If your concern is on traffic. Then you shouldn't worry much about the database, you should however worry about how your entire infrastructure scales.

Anyway considering you have your infrastructure figured out, then you might consider the following.

There are many considerations to  choosing the right database. Most people here are just fanboys of their technologies, and think whatever they use is the best! I find that to be very boring and uninformative.

Database considerations:

1- Price ( If you're willing to pay tons of money then go with msql or oracle)

2- Support ( related to price; however, open-source databases have enormous communities that you can find help and support for many common problems)

3- Security ( almost all dbs today are well secured )

4- Transaction Volume

5- Application ( your data should be driven by your application, see how you application retrieves and stores your data and decide which is better for you application)

Moreover, you have nosql (document) databases; such as, mongodb, couchdb, etc. They are not your conventional sql databases, but they excel in non-relational mapping such as in facebook and/or twitter where you can find out friends of your friends and so on.

Lastly, I generally hate general question. Technical answers need technical question. You're shooting yourself in the foot by asking these general questions where - I assure you - almost everyone in the world has a different answer. 

First go and ask our old little friend Google. Understand what you want, Research what are the current trends in whatever subject wither databases or whatever. Then come and ask a specific question to get a specific answer.

Good luck!

all databases are relatively the same, it depend of the staff skills and the familiarity with the database, Oracle is the most complex and designed for huge volume of data, yet we see companies like Facebook using something like MySQL so it's all based on your team ability to deal with the challenges that each database has, second is support, companies like Microsoft and Oracle support their own products and care about patching the issues as soon as they find it, others which is open-source based it may all depend on your team, unless you hire or make a contract with a company that take care of it, you need to have a look at your team, budget, system load and the future improvement/size .... which means use the one that suite your business and never forget to think about future changes and take them into account.

if we talk in the number of users  that can connect to database ,i think that Oracle is the most performan and when many users manipulate your database, you must secures it and oracle is important in this domain

Yes it depends on how much you want your database to be secured.

Because Oracle database is most secured and and less complexity. So, during the design phase you have to analyze what is the need of application and what we are going to build.