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In this article, we break down a complex SQL query that modifies the id values of rows in the Seat table based on specific conditions, and sorts the resulting data. Let's walk through each component of the query to understand how it functions.

1. Subquery (COUNT Calculation)

The subquery calculates the total number of rows in the Seat table:

sql

(SELECT 
    COUNT(*) AS counts
FROM
    Seat) AS seat_counts

This query counts all the rows in the Seat table and returns the total number as counts. This value is used in the main query for comparison with the id of each row.

2. Main Query with CASE Statement

The main query selects two columns: a computed id and the student from the Seat table. The computation of id is done using a CASE statement:

sql

SELECT 
    (CASE
        WHEN MOD(id, 2) <> 0 AND counts != id THEN id + 1
        WHEN MOD(id, 2) <> 0 AND counts = id THEN id
        ELSE id - 1
    END) AS id,
    student

The CASE statement applies the following logic to each row:

• Odd id and id is not the last row: The id is incremented by 1.
• Odd id and id is the last row: The id remains unchanged.
• Even id: The id is decremented by 1.

This transformation ensures that the final id values are adjusted based on whether they are odd or even and whether they match the total count of rows in the Seat table.

3. Cartesian Product

The FROM clause generates a Cartesian product of the Seat table and the result of the subquery:

sql

FROM
    Seat,
    (SELECT 
        COUNT(*) AS counts
    FROM
        Seat) AS seat_counts

This step essentially joins the Seat table with the total count of rows, allowing the counts value to be available for each row in the Seat table for comparison in the CASE statement.

4. Ordering the Results

Finally, the results are ordered by the newly computed id values:

sql

ORDER BY id ASC;

This clause ensures that the rows are presented in ascending order based on the adjusted id values.

Conclusion

The purpose of this query is to adjust the id of each row in the Seat table based on whether it is odd or even and whether it matches the total row count. The final result is then sorted by these newly calculated id values. This query can be useful in seating arrangements, reassigning seat numbers, or other similar use cases where identifiers need to be modified based on specific conditions.