join three tables

[William Lou]

I have a problem about joining three tables A, B, C:
table A has the serial_number as its primary key,
table B has the serial_number as its primary key but serial_number is not unique in this table (combined with category, group etc. other columns as primary key together )
table C has some common columns with table B

Now I want to select the serial_number from table B with certain conditions from Table C, so I could use the following query:
select distinct tB.serial_number from table_B as tB join table_C as tC Where Conditions;

And then, I would like to add the serial_number in table_A but not in table B to the serial_number I just selected, so I would use something like:
select distinct tB.serial_number from table_B as tB join table_C as tC Where Conditions
UNION
select serial_number from table_A not in table_B;

is it right? I think it's the right way to do that.

Another question, with the same task, can I do the query like this:
select tA.serial_number from table_A as tA LEFT JOIN table_B as tB ON tA.serial_number = tB.serial_number JOIN tableC as tC WHERE Conditions..

That's what I understand about "LEFT JOIN", SQL will select serial_number from table_B and table_C with the conditions satisfied first and then join table_A with serial_number only in table_A but not in table_B. Am I right?

Thanks for your reply!!!!!