user: Question: What is the production code for the episode written by Drew Z. Greenberg?
Columns: ['Episode No.', 'Title', 'Directed by', 'Written by', 'Original air date', 'Production code']
Types: ['real', 'text', 'text', 'text', 'text', 'text']
Sample row: [1.0, '" Serenity "', 'Joss Whedon', 'Joss Whedon', 'December 20, 2002', '1AGE79']
SQL:

assistant: SELECT "Production code" FROM "Table" WHERE "Written by" = 'Drew Z. Greenberg';