CREATE TABLE table_10592536_8 (date_of_appointment VARCHAR2, date_of_vacancy VARCHAR2)

What is the date of appointment when the date of vacancy is 21 december 2007?
SELECT date_of_appointment FROM table_10592536_8 WHERE date_of_vacancy = "21 December 2007"
SELECT "date_of_appointment" FROM "table_10592536_8" WHERE "21 December 2007" = "date_of_vacancy"