DEFINE invoice_count = SELECT COUNT(Invoice#) from INVOICE NUMBER;

This doesnt work. What's the correct syntax or method for doing this in SQL*Plus?

Hi,

you have to use a sql*plus variable.

Example:
<SQL>
-- Declare the variable INVOICE_COUNT as number
variable invoice_count number

-- fill the variable in a anonym-plsql-block
begin
SELECT COUNT(Invoice#)
into :invoice_count
from INVOICE NUMBER;
end;
/

-- now you can output the Value
print invoice_count

-- or us it in a subsequence select
select * from test_table where column1=:invoice_count;

</SQL>