Sql – How to find schema name in Oracle ? when you are connected in sql session using read only user

To see all tables in another schema, you need to have one or more of the following system privileges:

SELECT ANY DICTIONARY
(SELECT | INSERT | UPDATE | DELETE) ANY TABLE

or the big-hammer, the DBA role.

With any of those, you can select:

SELECT DISTINCT OWNER, OBJECT_NAME 
  FROM DBA_OBJECTS
 WHERE OBJECT_TYPE = 'TABLE'
   AND OWNER = '[some other schema]'

Without those system privileges, you can only see tables you have been granted some level of access to, whether directly or through a role.

SELECT DISTINCT OWNER, OBJECT_NAME 
  FROM ALL_OBJECTS
 WHERE OBJECT_TYPE = 'TABLE'
   AND OWNER = '[some other schema]'

Lastly, you can always query the data dictionary for your own tables, as your rights to your tables cannot be revoked (as of 10g):

SELECT DISTINCT OBJECT_NAME 
  FROM USER_OBJECTS
 WHERE OBJECT_TYPE = 'TABLE'

I suspect you created the table using double quotes using e.g. "Clients" or some other combination of upper/lowercase characters and therefor the table name is case sensitive now.

What does the statement

 SELECT table_schema, table_name
 FROM information_schema.tables 
 WHERE lower(table_name) = 'clients'

return?

If the table name that is returned is not lowercase you have to use double quotes when referring to it, something like this:

String query = "SELECT * FROM \"Clients\"";