Understanding the Problem

SQL Query - SUM function returning wrong result

In this article, we will delve into the complexities of SQL queries and explore how to correctly calculate the sum of areas for lakes that contain at least one island for each continent.

The problem statement involves generating a table with continents and their respective lake area shares. To do this, we need to join multiple tables: IslandIn, Lake, and geo_lake. The query provided attempts to achieve this by using Common Table Expressions (CTEs) and joins.

However, there are several issues with the query that lead to incorrect results. In this article, we will break down these problems and provide a corrected solution.

ER Diagram and Relational Schema

To understand the database schema better, let’s first review the provided ER diagram and relational schema:

The ER diagram shows the following relationships between tables:

• An IslandIn table with an id, lake_id, and country_id.
• A Lake table with an id and name.
• A geo_lake table with a lake_id and country.

The relational schema is defined as follows:

CREATE TABLE IslandIn (
    id INT PRIMARY KEY,
    lake_id INT,
    country_id INT
);

CREATE TABLE Lake (
    id INT PRIMARY KEY,
    name VARCHAR(255)
);

CREATE TABLE geo_lake (
    lake_id INT,
    country INT,
    FOREIGN KEY (lake_id) REFERENCES Lake(id),
    FOREIGN KEY (country) REFERENCES IslandIn(country)
);

The provided SQL query attempts to join these tables and calculate the sum of areas for lakes that contain at least one island for each continent. However, there are several issues with this approach.

Issues with the Original Query

1. Incorrect Join Order

The original query joins LakesWithIslands (a CTE) with encompasses, which is not a standard table name in PostgreSQL. The correct table name should be geography, as per the provided ER diagram and relational schema.

SELECT B.continent, (B.percentage/100)*SUM(A.area)
FROM LakesWithIslands as A
INNER JOIN geography as B ON A.country = B.country

2. Incorrect Join Condition

The original query joins LakesWithIslands with encompasses based on the country column. However, this join condition is incorrect because it assumes that a country belongs to multiple continents, which is not the case.

Instead, we need to join LakesWithIslands with the Lake table based on the lake_id, and then join this result with the geo_lake table using the country_id.

SELECT B.continent, (B.percentage/100)*SUM(A.area)
FROM LakesWithIslands as A
INNER JOIN Lake AS B ON A.lake = B.name
INNER JOIN geo_lake AS C ON A.lake = C.lake_id
GROUP BY B.continent, B.percentage

3. Lack of Continent Information

The original query does not include the continent information in the LakesWithIslands CTE. To fix this, we need to join LakesWithIslands with the geography table based on the country.

SELECT B.continent, (B.percentage/100)*SUM(A.area)
FROM (
  SELECT DISTINCT IslandIn.lake, Lake.area, geography.country
  FROM ((IslandIn
        INNER JOIN Lake ON IslandIn.lake = Lake.name)
             INNER JOIN geo_lake ON IslandIn.lake = geo_lake.lake_id)
       WHERE IslandIn.lake IS NOT NULL
) as A
INNER JOIN geography AS B ON A.country = B.country
GROUP BY B.continent, B.percentage

Corrected Query

Here is the corrected SQL query that calculates the sum of areas for lakes that contain at least one island for each continent:

SELECT B.continent, (B.percentage/100)*SUM(A.area)
FROM (
  SELECT DISTINCT IslandIn.lake, Lake.area, geography.country
  FROM ((IslandIn
        INNER JOIN Lake ON IslandIn.lake = Lake.name)
             INNER JOIN geo_lake ON IslandIn.lake = geo_lake.lake_id)
       WHERE IslandIn.lake IS NOT NULL
) as A
INNER JOIN geography AS B ON A.country = B.country
GROUP BY B.continent, B.percentage

This query joins LakesWithIslands with the geography table based on the country, and then groups the result by continent. The final calculation is performed using the correct join order and conditions.

Conclusion

Calculating the sum of areas for lakes that contain at least one island for each continent can be a challenging task, especially when dealing with complex database schema and relationships.

In this article, we have broken down the problems with the original query and provided a corrected solution. The key takeaways from this article are:

• Correct join order is crucial in SQL queries.
• Join conditions must be accurate to avoid incorrect results.
• Continent information is essential for correct calculations.
• Using Common Table Expressions (CTEs) can simplify complex queries.

By following these guidelines and using the corrected query provided in this article, you should be able to accurately calculate the sum of areas for lakes that contain at least one island for each continent.

Last modified on 2025-03-27