INSERT INTO test_table (avail_seq) VALUES ((SELECT
COALESCE(MAX(avail_seq), 0) + 1 FROM test_table));

Whenever I run that SQL query, I get the following error:

The name 'avail_seq' is illegal in this context. Only constants,
constant expressions, or variables allowed here. Column names are
illegal. (severity 15, procedure N/A)

Here's my table definition:

create table test_table (
avail_seq            int           not null,

constraint test_table primary key (avail_seq)
)
grant all on test_table to public;

It seems to me like using IDENTITY would solve this, but alas, I
didn't create the table.

An alternative would be to do the SELECT subquery, by itself, and then
the INSERT, later, but that wouldn't be atomic.

In MySQL, I could use transactions.  ie.

BEGIN;
@avail_seq = SELECT COALESCE(MAX(avail_seq), 0) + 1 FROM test_table;
INSERT INTO test_table (avail_seq) VALUES (@avail_seq);
COMMIT;

Or something like that.  Unfortunately, I'm not sure how to do that
with Sybase.

Any ideas?