在employee表中有以下信息{ "____id" : 1, "姓名" : "张飞", "性别" : "male", "年龄" : 18, "工号" : 20170301, "工资" : 7300.33, "职位" : "teacher","部门号" : 401 }{ "____id" : 2, "姓名" : "曹操", "性别" : "male", "年龄" : 28, "工号" : 20121101, "职位" : "teacher", "工资" : 2100, "部门号" : 401 }{ "____id" : 3, "姓名" : "诸葛亮", "性别" : "female", "年龄" : 18, "工号" : 20110211, "职位" : "teacher", "工资" : 9000, "部门号" : 401 }{ "____id" : 4, "姓名" : "周瑜", "性别" : "male", "年龄" : 18, "工号" : 19000301, "职位" : "teacher", "工资" : 30000, "部门号" : 401 }{ "____id" : 5, "姓名" : "司马懿", "性别" : "male", "年龄" : 48, "工号" : 20101111, "职位" : "teacher", "工资" : 10000, "部门号" : 401 }{ "____id" : 6, "姓名" : "袁绍", "性别" : "female", "年龄" : 48, "工号" : 20150311, "职位" : "sale", "工资": 3000.13, "部门号" : 402 }{ "____id" : 7, "姓名" : "张全蛋", "性别" : "female", "年龄" : 38, "工号" : 20101101, "职位" : "sale", "工资" : 2000.35, "部门号" : 402 }{ "____id" : 8, "姓名" : "曹操", "性别" : "female", "年龄" : 18, "工号" : 20110312, "职位" : "sale", "工资": 1000.37, "部门号" : 402 }{ "____id" : 9, "姓名" : "孙权", "性别" : "female", "年龄" : 18, "工号" : 20160513, "职位" : "sale", "工资": 3000.29, "部门号" : 402 }{ "____id" : 10, "姓名" : "貂蝉", "性别" : "female", "年龄" : 28, "工号" : 20170127, "职位" : "sale", "工资": 4000.33, "部门号" : 402 }计算职位为sale的员工的平均年龄