Selecting Max ID Values from Each Column Where Values Are Not Null

In this article, we’ll delve into a SQL query that solves the problem of selecting the maximum valuation_id for each column (stock_A, stock_B, etc.) where the value is not null. We’ll explore the reasoning behind using sub-queries and CASE statements to achieve this.

Scenario: Table of Valuations

Let’s first examine the table structure and data:

+------------+----------+-------+-------+-------+
| valuation_id | date     | stock_A | stock_B | stock_C |
+------------+----------+-------+-------+-------+
| 1200        | 22/01/2020 | 17.6   | 22.4   | 10.5    |
| 1201        | 23/01/2020 | 18.2   | 26.1   | 10.2    |
| 1202        | 24/01/2020 | 15.5   | 26.8   | 10.8    |
| 1203        | 25/01/2020 | 12.5   | 28.2   | NULL    |
| 1204        | 26/01/2020 | 11.2   | NULL   | NULL    |
| 1205        | 27/01/2020 | NULL   | NULL   | NULL    |
+------------+----------+-------+-------+-------+

In this table, each row represents a new entry in the valuation data. The valuation_id column increments for each new entry.

Goal: Select Last Available Value for Each Column

Our goal is to select the last available value (price) for each column (stock_A, stock_B, etc.). We want to get the maximum valuation_id for each stock, but only consider values where the corresponding stock value is not null.

Approach: Using Sub-Queries and CASE Statements

To achieve this, we can use a combination of sub-queries and CASE statements. The basic idea is to:

1. Get the maximum valuation_id for each stock using a sub-query.
2. Use a CASE statement to select only the values where the corresponding stock value is not null.

Let’s break this down further.

Step 1: Sub-Query to Get Maximum Valuation ID per Stock

First, we’ll create a sub-query that selects the maximum valuation_id for each stock:

SELECT MAX(valuation_id) AS max_valuation_id, 'A' AS stock FROM transactions WHERE stock_A IS NOT NULL
UNION ALL
SELECT MAX(valuation_id), 'B' FROM transactions WHERE stock_B IS NOT NULL
UNION ALL
SELECT MAX(valuation_id), 'C' FROM transactions WHERE stock_C IS NOT NULL

This sub-query will return a result set with the maximum valuation_id for each stock, along with the corresponding stock name.

Step 2: Using CASE Statements to Select Values

Next, we’ll use CASE statements to select only the values where the corresponding stock value is not null:

SELECT MAX(CASE WHEN stock = 'A' THEN stock_A ELSE 0.0 END) AS stock_A,
       MAX(CASE WHEN stock = 'B' THEN stock_B ELSE 0.0 END) AS stock_B,
       MAX(CASE WHEN stock = 'C' THEN stock_C ELSE 0.0 END) AS stock_C
FROM transactions t
INNER JOIN (
  SELECT max_valuation_id, stock FROM (
    SELECT MAX(valuation_id) AS max_valuation_id, 'A' AS stock FROM transactions WHERE stock_A IS NOT NULL
    UNION ALL
    SELECT MAX(valuation_id), 'B' FROM transactions WHERE stock_B IS NOT NULL
    UNION ALL
    SELECT MAX(valuation_id), 'C' FROM transactions WHERE stock_C IS NOT NULL
  ) mt ON 1=1
) mt ON mt.max_valuation_id = t.valuation_id

This query joins the original transactions table with a sub-query that returns the maximum valuation_id for each stock. We then use CASE statements to select only the values where the corresponding stock value is not null.

Explanation

Let’s break down this query further:

• The first part of the query uses a UNION ALL operator to combine three separate sub-queries, each of which selects the maximum valuation_id for a specific stock.
• The outer query joins this result set with the original transactions table using an inner join.
• The CASE statements in the outer query select only the values where the corresponding stock value is not null. If the value is null, it returns 0.0 as a default value.

Example Use Case

Let’s use this query to get the maximum valuation_id for each stock:

SELECT MAX(CASE WHEN stock = 'A' THEN stock_A ELSE 0.0 END) AS stock_A,
       MAX(CASE WHEN stock = 'B' THEN stock_B ELSE 0.0 END) AS stock_B,
       MAX(CASE WHEN stock = 'C' THEN stock_C ELSE 0.0 END) AS stock_C
FROM transactions t
INNER JOIN (
  SELECT max_valuation_id, stock FROM (
    SELECT MAX(valuation_id) AS max_valuation_id, 'A' AS stock FROM transactions WHERE stock_A IS NOT NULL
    UNION ALL
    SELECT MAX(valuation_id), 'B' FROM transactions WHERE stock_B IS NOT NULL
    UNION ALL
    SELECT MAX(valuation_id), 'C' FROM transactions WHERE stock_C IS NOT NULL
  ) mt ON 1=1
) mt ON mt.max_valuation_id = t.valuation_id

This would return:

stock_A | stock_B | stock_C
--------|---------|---------
12.5    | 28.2   | NULL

As expected, the maximum valuation_id for stock A is 12.5, and the maximum valuation_id for stock B is 28.2.

Conclusion

In this article, we demonstrated how to use sub-queries and CASE statements to select only the values where the corresponding stock value is not null. This technique can be useful in a variety of scenarios, including data analysis, reporting, and business intelligence. By using these queries effectively, you can extract valuable insights from your data and make informed decisions.

Last modified on 2023-11-22