Why companies ask for mysql,oracle sql knowledge when there is a separate team for that and what is the work of system administrator on the databases like mysql and sql

It is extremely beneficial for a systems admin to have at least a working knowledge of the applications that are running on the servers they are administering.

A basic knowledge of things like which port MySQL runs on, how to check that the server is running, how to run simple queries, how to install and secure a MySQL installation are things I'd consider essential when hiring. The same for apache.

Even if companies have "teams" for these it is highly desirable to have a sysadmin that can understand when these teams talk about MySQL master/slave replication, circular replication, or apache name based virtual hosting and IP based virtual hosting.

May be as an Admin,
1. Update new patches
2. Troubleshoot the issues
3. Fine tune the server performance
4. Grant permission to users/clients
5. Manager cluster resources
6. Manage the backup/ restore the backup
7. Logging the slow queries
8. Mainly DB team think about to build database
9. As an Admin need to think about day to day management and High availability of the DB server
10. Etc..

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Think of it like a car, you don't need to know how to build an engine to drive a car but you should know if it uses petrol or diesel and also where to put these in!

Even though "DBAs Are Gods,™" those Gods do need to be able to talk to Mortals who have a vague notion of what they are doing. And also, "one does not wish to bother The Gods for trivial matters." Therefore, to the degree that "day-to-day administration of the platform" logically involves routine activities regarding the databases, or routine diagnosis of problems (say, in TCP/IP connections) as they crop up from time to time, a sysadmin needs to have a fair amount of understanding of this stuff. (Which understanding must also include "knowing where the deep water starts" and having the lack-of-hubris needed to stay out of there.)

For the americans: he means gas/gasoline.

As a Sysop, I want to know how to administer anything that runs on my machines. Anything that has a config file, log file and init.d startup script is something that I go out of my way to know about for the inevitable day that it breaks.

Mysql can be installed using packages in the cd or we can download the rpm from the website why do we need to do that when we have mysql rpm in the dvd

...and....
These questions would seem to indicate that you're either a brand-new systems administrator, or you haven't yet got a job as one.

Companies can ASK for whatever they want...if they want you to have a doctorate degree in ancient Greek, you'd better have one. Why? Because they WANT IT, that's why. Some other reasons, which an administrator with experience would point out would be:

• Your servers may be running MySQL...shouldn't you be able to troubleshoot the software ON the server?
• Managing MySQL may be part of your job.
• The DBA 'team' may consist of one person at some companies, and 50 at another...who knows?
• Many applications use MySQL as a back end for data, so the DBA's won't be touching it...just the systems administrators.

...which goes to your next question, about why you would install it from a website/repository. If MySQL has 10 updates between when the DVD is released and the time you've installed your new server with it, wouldn't you WANT the latest package???

It can also help to identify the idiots that want to break the systems security just because "they need to get their work done"...

It helps when the DBA is throwing crap at the system and the system quits - and blames the admin for their failings.

Always know more than you need - you never know when it will save your ass.

Let me put it this way.. It's not a system admins job to fix bugs, but it is partially his job to find some of those bugs...

Here's why: A sysadmin doesn't only install stuff, but it makes sure everything works together and when they don't (and trust me, they ussualy don't) it's his job to find out why (and if he can, to fix it -- if he can't, to report it).. You can't debug something without knowing nothing about it..

Here's another example: How will you update your DB server not knowing SQL? Just running the provided scripts? Pff.. Sure, databases and queries never brake :P Hell, a sysadmin will probably know more about the database software than most of the developers..