Java开发中，先根据离职时间利用sql查出在起始年月到结束年月区间的离职人员，并且省份包含上海、广州、深圳。如何利用lambda表示再查询出每个省份每个月离职率

假设有一张名为employee的表，其中有以下字段：id, name, province, leave_date。

先根据离职时间查出在起始年月到结束年月区间的离职人员：

SELECT * FROM employee WHERE leave_date BETWEEN '起始年月' AND '结束年月'

筛选出省份包含上海、广州、深圳的离职员工：

SELECT * FROM employee WHERE leave_date BETWEEN '起始年月' AND '结束年月' AND province IN ('上海', '广州', '深圳')

利用lambda表达式查询出每个省份每个月的离职率：

List<Employee> employees = ...; // 假设为查询结果集

// 以省份和月份为键，统计离职次数
Map<String, Map<String, Integer>> leaveCounts = employees.stream()
    .collect(Collectors.groupingBy(Employee::getProvince, 
             Collectors.groupingBy(emp -> emp.getLeaveDate().substring(0, 7), Collectors.summingInt(emp -> 1))));

// 以省份和月份为键，统计总人数
Map<String, Map<String, Integer>> totalCounts = employees.stream()
    .collect(Collectors.groupingBy(Employee::getProvince, 
             Collectors.groupingBy(emp -> emp.getLeaveDate().substring(0, 7), Collectors.summingInt(emp -> 1))));

// 计算每个省份每个月的离职率
Map<String, Map<String, Double>> leaveRates = new HashMap<>();
for (String province : leaveCounts.keySet()) {
    Map<String, Integer> provinceLeaveCounts = leaveCounts.get(province);
    Map<String, Integer> provinceTotalCounts = totalCounts.get(province);
    for (String month : provinceTotalCounts.keySet()) {
        double rate = provinceLeaveCounts.getOrDefault(month, 0) / (double) provinceTotalCounts.get(month);
        leaveRates.computeIfAbsent(province, k -> new HashMap<>()).put(month, rate);
    }
}

其中Employee类的定义为：

public class Employee {
    private int id;
    private String name;
    private String province;
    private String leaveDate;
    
    // 省略构造方法、getter和setter
}