Newbie select questions

We are running an evaluation copy of oracle, which explains the newbie part, and I am having problem understanding the following:

Depending on which oracle login I am using, I am seeing different things. To be specific, the following is an example:

        If I am logged in as A, I do a:

                select count(*) from ALL_TABLES;

	I will get, say, 70. I keep getting totally different numbers
	if I am logged in as B, C, or whatever else!

Suspecting privilege was the culprit, I went looking at TABLE_PRIVILEGES for ALL_TABLES and got the following:

	TABLE_NAME	OWNER	GRANTOR	GRANTEE		S I D U R A I C
	--------------- ------- ------- -------         - - - - - - - -
	ALL_TABLES	SYS	SYS	PUBLIC		G N N N N N N

I suppose Y is for Yes and N is for No, but what in the world does G stand for?

My other problems include not being able to select on views which has PUBLIC in the GRANTEE column and a Y in the S column of the TABLE_PRIVILEGES table.

What more does one need to select?

Totally puzzled at this point. Any help will be greatly appreciated.

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chan_at_oit.umass.edu