将MySQL查询移植到PostgreSQL

Question

I'm migrating my MySQL databases to PostgreSQL. While connecting database to my Python script I'm getting some syntax error. So could you please convert this MySQL query into PostgreSQL:

select ifnull((select ifnull(run_id,0) from searchbank order by run_id desc limit 0,1),0)

Answer 1

Postgres doesn't support ifnull:

select coalesce((select coalesce(run_id,0)
                 from searchbank
                 order by run_id desc limit 0,1),0
               )

You might find this version slightly cleaner and clearer:

select coalesce(max(run_id), 0)
from searchbank

Answer 2

Simpler:

SELECT COALESCE((SELECT max(run_id) FROM searchbank), 0);

The only point of a query like this is to return 0 instead of no row if searchbank has no rows.

Removing the outer SELECT - like @Gordon demonstrates - cancels that effect and is strictly wrong.