c# first time working with database

hi guys,
 
im for first time using a database in c#.
I would like to know how do i read, compare values
in different data tables.
 
And most important, how do i get all values of a row from a datatable.
I have 4 DataTables. In ech table are some rows. Now i get that
rows into a form. By doing this, i have created relations to the tables from
the dataset.
 
After that, i opened the dataset. I have seen that it is possible on "right-click"
to add a method for searching/output values from tables, querry.
 
Now i started the querry helper, by clicking my table and set the method settings,
for searching my value. Afterwards i opened my form and set the Table and the
table adapter. I searched that table with "foreach". Everything works fine.
 
But, now i need to search another table, and by doing this by the same proces
as explained it doesnt work, it tells me on creating the query, the
shema is another as the main query.
 
Let me know if you need a more precise explaination.
 
Thank you
 
Greets Niko