join problem

I have join problem:
"select i.ivid, v.eventdate, v.deposit, v.invdate, cai.db,
sum(i.tax) as tax,
sum(i.tax + i.rowtot) as totalP,
(sum(i.tax + i.rowtot) - v.deposit) as balance
from invoice v
left outer join
invoiceitems i
on v.ivid = i.ivid
where v.cusid = $cusid
and v.cusid = cai.cusid
group by i.ivid, v.eventdate, v.deposit, v.invdate, cai.db
ERROR: missing FROM-clause entry for table "cai"

If I add cai to the from clause "from invoice v, cai, I get
ERROR: missing FROM-clause entry for table "cai"
ERROR: invalid reference to FROM-clause entry for table "v"

Where do I add the cai table reference???

thanks

--
Arthur R. Van Hook
Mayor - Retired
The City of Lake Lotawana

hook(at)lake-lotawana(dot)mo(dot)us
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