Untitled

Let’s break down the provided SQL query step by step to understand how it works. The query is designed to calculate the sum of tiv_2016 for policyholders who meet two specific conditions:

1. They share the same tiv_2015 value with at least one other policyholder.
2. Their (lat, lon) location is unique (i.e., no other policyholder shares the same location).

The query uses Common Table Expressions (CTEs) to simplify the logic. Here’s a detailed explanation of each part:

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1. unique_locations CTE

WITH unique_locations AS (
    SELECT
        lat,
        lon
    FROM
        Insurance
    GROUP BY
        lat,
        lon
    HAVING
        COUNT(*) = 1
)

Purpose:

This CTE identifies all unique (lat, lon) pairs in the Insurance table. A location is considered unique if it appears only once in the table.

How it works:

• The GROUP BY lat, lon groups rows by their (lat, lon) pairs.
• The HAVING COUNT(*) = 1 filters the groups to include only those where the count of rows is exactly 1. This ensures that only unique locations are selected.

Output:

A list of (lat, lon) pairs that appear only once in the Insurance table.

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2. shared_tiv_2015 CTE

shared_tiv_2015 AS (
    SELECT
        tiv_2015
    FROM
        Insurance
    GROUP BY
        tiv_2015
    HAVING
        COUNT(*) > 1
)

Purpose:

This CTE identifies all tiv_2015 values that are shared by at least two policyholders.

How it works:

• The GROUP BY tiv_2015 groups rows by their tiv_2015 values.
• The HAVING COUNT(*) > 1 filters the groups to include only those where the count of rows is greater than 1. This ensures that only tiv_2015 values shared by multiple policyholders are selected.

Output:

A list of tiv_2015 values that appear more than once in the Insurance table.

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3. Main Query

SELECT
    ROUND(SUM(tiv_2016)::numeric, 2) AS tiv_2016
FROM
    Insurance
WHERE
    tiv_2015 IN (SELECT tiv_2015 FROM shared_tiv_2015)
    AND (lat, lon) IN (SELECT lat, lon FROM unique_locations);

Purpose:

This part of the query calculates the sum of tiv_2016 for policyholders who meet both conditions:

1. Their tiv_2015 value is shared with at least one other policyholder.
2. Their (lat, lon) location is unique.

How it works:

1. Filtering by tiv_2015:

   • The WHERE tiv_2015 IN (SELECT tiv_2015 FROM shared_tiv_2015) condition ensures that only policyholders with a tiv_2015 value that appears in the shared_tiv_2015 CTE are included. These are policyholders who share their tiv_2015 value with at least one other policyholder.

2. Filtering by Unique Locations:

   • The AND (lat, lon) IN (SELECT lat, lon FROM unique_locations) condition ensures that only policyholders with a unique (lat, lon) pair (as identified in the unique_locations CTE) are included.

3. Summing and Rounding tiv_2016:

   • The SUM(tiv_2016) calculates the total tiv_2016 for all policyholders who meet the above conditions.
   • The ROUND(SUM(tiv_2016)::numeric, 2) rounds the sum to two decimal places.

Output:

The query returns a single value: the sum of tiv_2016 for all policyholders who meet the criteria, rounded to two decimal places.

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Example Walkthrough

Input Table (Insurance):

pid	tiv_2015	tiv_2016	lat	lon
1	224.17	952.73	32.4	20.2
2	224.17	900.66	52.4	32.7
3	824.61	645.13	72.4	45.2
4	424.32	323.66	12.4	7.7
5	424.32	282.9	12.4	7.7
6	625.05	243.53	52.5	32.8
7	424.32	968.94	72.5	45.3
8	624.46	714.13	12.5	7.8
9	425.49	463.85	32.5	20.3
10	624.46	776.85	12.4	7.7

Step 1: unique_locations CTE

• Identifies unique (lat, lon) pairs:
  • (32.4, 20.2)
  • (52.4, 32.7)
  • (72.4, 45.2)
  • (52.5, 32.8)
  • (12.5, 7.8)
  • (32.5, 20.3)

Step 2: shared_tiv_2015 CTE

• Identifies shared tiv_2015 values:
  • 224.17 (shared by pid 1 and 2)
  • 424.32 (shared by pid 4, 5, and 7)
  • 624.46 (shared by pid 8 and 10)

Step 3: Main Query

• Filters policyholders who:
  • Have tiv_2015 in (224.17, 424.32, 624.46).
  • Have a unique (lat, lon) pair.
• Sums their tiv_2016 values and rounds the result to two decimal places.

Output:

tiv_2016
8206.2

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Key Points

1. CTEs Simplify Logic: The use of CTEs (unique_locations and shared_tiv_2015) makes the query modular and easier to understand.
2. Filtering Conditions: The WHERE clause ensures that only policyholders who meet both conditions are included in the final calculation.
3. Rounding: The ROUND function ensures the result is formatted to two decimal places.

This query is efficient and correctly solves the problem as described. Let me know if you have further questions!