user: Question: Who is the incoming manager when the date of vacancy was 21 March 2011?
Columns: ['Team', 'Outgoing manager', 'Manner of departure', 'Date of vacancy', 'Incoming manager', 'Date of appointment', 'Position in Table']
Types: ['text', 'text', 'text', 'text', 'text', 'text', 'text']
Sample row: ['Hereford United', 'Graham Turner', 'Resigned', '16 April 2010', 'Simon Davey', '22 June 2010', 'Pre-Season']
SQL:

assistant: SELECT "Incoming manager" FROM "Table" WHERE "Date of vacancy" = '21 March 2011';