【練習一】Write a query to print the sum of all total investment values in 2016 (TIV_2016), to a scale of 2 decimal places, for all policy holders who meet the following criteria:

1. Have the same TIV_2015 value as one or more other policyholders.
2. Are not located in the same city as any other policyholder (i.e.: the (latitude, longitude) attribute pairs must be unique).

Input Format:
The insurance table is described as follows:

| Column Name | Type          |
|-------------|---------------|
| PID         | INTEGER(11)   |
| TIV_2015    | NUMERIC(15,2) |
| TIV_2016    | NUMERIC(15,2) |
| LAT         | NUMERIC(5,2)  |
| LON         | NUMERIC(5,2)  |

where PID

Sample Input

| PID | TIV_2015 | TIV_2016 | LAT | LON |
|-----|----------|----------|-----|-----|
| 1   | 10       | 5        | 10  | 10  |
| 2   | 20       | 20       | 20  | 20  |
| 3   | 10       | 30       | 20  | 20  |
| 4   | 10       | 40       | 40  | 40  |

Sample Output

| TIV_2016 |
|----------|
| 45.00    |

SELECT
    SUM(insurance.TIV_2016) AS TIV_2016
FROM
    insurance
WHERE
    insurance.TIV_2015 IN
    (
      SELECT
        TIV_2015
      FROM
        insurance
      GROUP BY TIV_2015
      HAVING COUNT(*) > 1
    )
    AND CONCAT(LAT, LON) IN
    (
      SELECT
        CONCAT(LAT, LON)
      FROM
        insurance
      GROUP BY LAT , LON
      HAVING COUNT(*) = 1
    )
;

 【練習二】A university uses 2 data tables, student and department, to store data about its students and the departments associated with each major.

Write a query to print the respective department name and number of students majoring in each department for all departments in the department table (even ones with no current students).

Sort your results by descending number of students; if two or more departments have the same number of students, then sort those departments alphabetically by department name.

The student is described as follow:

| Column Name  | Type      |
|--------------|-----------|
| student_id   | Integer   |
| student_name | String    |
| gender       | Character |
| dept_id      | Integer   |

where student_id is the student's ID number, student_name is the student's name, gender is their gender, and dept_id is the department ID associated with their declared major.

And the department table is described as below:

| Column Name | Type    |
|-------------|---------|
| dept_id     | Integer |
| dept_name   | String  |

where dept_id is the department's ID number and dept_name is the department name.

Here is an example input:
student table:

| student_id | student_name | gender | dept_id |
|------------|--------------|--------|---------|
| 1          | Jack         | M      | 1       |
| 2          | Jane         | F      | 1       |
| 3          | Mark         | M      | 2       |

department table:

| dept_id | dept_name   |
|---------|-------------|
| 1       | Engineering |
| 2       | Science     |
| 3       | Law         |

The Output should be:

| dept_name   | student_number |
|-------------|----------------|
| Engineering | 2              |
| Science     | 1              |
| Law         | 0              |

SELECT
    dept_name, COUNT(*) AS student_number
FROM
    department
        LEFT OUTER JOIN
    student ON department.dept_id = student.dept_id
GROUP BY department.dept_name
ORDER BY student_number DESC , department.dept_name
;