user: Question: What is ISBN UK, when Author is "Dave Martin", and when ISBN US is "n/a"?
Columns: ['Title', 'Author', 'ISBN UK', 'ISBN US', 'TV companions featured']
Types: ['text', 'text', 'text', 'text', 'text']
Sample row: ['Search for the Doctor', 'Dave Martin', 'ISBN 0-7278-2087-7', 'ISBN 0-345-33224-5', 'K-9']
SQL:

assistant: SELECT "ISBN UK" FROM "Table" WHERE "Author" = 'Dave Martin' AND "ISBN US" = 'n/a';