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problems can occur when we update system tables

What kinds of problems can occur when we update system tables in user databases?
Can any one give me a list of problems which might happen after we update system tables in users databaes say ‘sysusers’.
If I change the db status to single user mode will
these problem come?
Should I change the whole server to single user mode or just one db at a time.?
pls suggest
All changes that you may at any time need to do to data in system tables can be done more easily using system SPs. The syntax for calling these SPs is much easier than the syntax that you would need to write any sort of action query, and so you are bound to make mistakes. Also, there are lots of undocumented features of system tables, and so your action query may be missing critical details. So DON’T. Sometimes there is information that you cannot find with a simple query, like "which SPs contain the word JOHN", and then you may need to write SELECT queries against system tables. Try it, and you’ll notice things are not always what you expect. In this case, the syscomments table contains the script for all SPs, but any SP script can span multiple rows in the table. So a simple query won’t catch words that are broken off between rows. Plus you won’t be able to find words in encrypted procedures anyway.
Its not a good practice to update or query the system tables directly when there is a feature of pre-supplied stored procedures/functions. ANd if you update the system tables manually then the support contract with MS will be void.. if you have any. Its easy to keep database by database in single-user mode. Satya SKJ
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