並前往https://www.standrwslt.co/sql-%E4%B8%8D%E9%87%8D%E8%A4%87-count-sql-%E6%8C%89%E6%A2%9D%E4%BB%B6%E7%B5%B1%E8%A8%88%E9%9D%9E%E9%87%8D%E5%BE%A9%E5%80%BC%EF%BC%8Ccountdistinct/