假设就业数据表的结构如下所示：

CREATE TABLE employment (
  id INT PRIMARY KEY,
  employee_id INT,
  start_date DATE,
  end_date DATE
);

下面是一个使用SQL查询并计算服务时长总和的示例：

SELECT employee_id, SUM(DATEDIFF(end_date, start_date)) AS total_service_duration
FROM employment
GROUP BY employee_id;

上述查询将迭代就业数据表，并根据employee_id对服务时长进行求和，并将结果分组。请注意，上述示例基于SQL语言。如果您使用的是其他编程语言，例如Python，您可以使用相应的数据库驱动程序和语法来编写相似的查询。以下是使用Python和MySQL数据库驱动程序的示例代码：

import mysql.connector

conn = mysql.connector.connect(
  host="localhost",
  user="yourusername",
  password="yourpassword",
  database="yourdatabase"
)

cursor = conn.cursor()

query = """
SELECT employee_id, SUM(DATEDIFF(end_date, start_date)) AS total_service_duration
FROM employment
GROUP BY employee_id
"""

cursor.execute(query)

result = cursor.fetchall()

for row in result:
  employee_id = row[0]
  total_service_duration = row[1]
  print(f"Employee ID: {employee_id}, Total Service Duration: {total_service_duration} days")

conn.close()

请根据您实际使用的数据库和编程语言进行相应的更改。