SQL and Unix knowledge is must for tester......

Tester should have the knowledge of the application or the
requirements to a max. extent

Before answering this Questiong I would like to fisrt tell 
you few good qualities of a tester:

1) Test to break attitude.
2) Attention to details.
3) Good knowledge on the application being tested.
4) Understanding skills.
5) Ability to communicate with both technical (Developer's, 
Business analyst's) and non-technical (Customer's, 
Manager's) people.
6) Documentation skills

We can find the answer from the above line too :)

1)The first and fore most is he should be good at English.
so that he can be able to understand the requirements and 
can communicate with the non-technical people.
2)Good knowledge upon the application
3)Domain knowledge so that he can go in depth with the 
application and can communicate withthe technical people.
4)Documentation skills (helps mostly at the time of raising 
defects, so that next cycle person in the defect can 
understand what exactly defect is).

Actually the question's 1) Good qualities of tester
2) which knowledge is must for tester are almost answers 
the same.

Thanks.

a tester must know the domain knowledge of the project 
means about the functional behaviour of the project and the 
complete knowledge of the client requirement is must for 
the tester..

The first and foremost knowledge a tester should possess is 
the ability to look at things from different angles, which 
is anlytical skill. Knowledge about the domain is important 
but what is more important than that is how to use testing 
principles to effectively test what you understand from the 
domain.
Say your domain says, the minimum deposit for a customer of 
a bank is Rs.1000. Any one can derive a positive test case 
like testing for 1000. But the need to check less than 1000 
is crucial because that proves that the system may pass 
below this range, which should not be as per the domain.
Above example clearly points out...possessing domain 
knowledge and testing what you possess are two different 
skills although both are closely related to each other.

Domain knowledge is more important for a tester so that he 
can know how to test an application from all angles on 
system testing, network testing, functionality testing...

Should be Perfectionist
Should be tactful and diplomatic
Should be innovative and creative
Should be relentless
Should possess negative thinking with good judgment skills.
Should possess the attitude of break the system.

requirement of clients,and the knowledge of how we can make software defectless