Difficult query (for me) - Mailing list pgsql-sql

Hello,

Basicaly I have two tables like this:

Table 1
RecordName Value
a          100
b          100
c          100
d          100
e          100
f          100

Table 2
RecordName Value
a         25
b         50
c         75
g         150
h         150

I would like a query that give me a result like this:
a         75 (100-25)
b         50 (100-50)
c         25
d         100
e         100
f         100
g         -150
h         -150

It is table1-table2.
I could do it for records a,b,c but I do not how to have record which
are only in one of the two tables.
Thanks for your help.
Stephane.