Selecting a Specific Value from a Group Based on Order in MySQL/MariaDB

Here are two common approaches:

A. Using ROW_NUMBER():

This window function assigns a sequential number (starting from 1) to each row within a group defined by the GROUP BY clause. You can then filter based on the desired row number (e.g., 1 for the first row).

Example:

SELECT yourTable.id, yourTable.value
FROM yourTable
GROUP BY yourGroupingColumn
ORDER BY yourOrderColumn ASC  -- Order by the desired order column
HAVING ROW_NUMBER() OVER (PARTITION BY yourGroupingColumn ORDER BY yourOrderColumn ASC) = 1;

This query selects the id and value from the first row (based on yourOrderColumn ascending order) within each group defined by yourGroupingColumn.

B. Using LIMIT with Subquery:

This approach involves a subquery that retrieves the desired row based on the order. The main query then uses LIMIT 1 to fetch only that specific row.

SELECT yourTable.id, yourTable.value
FROM yourTable
WHERE yourTable.id IN (
  SELECT id
  FROM (
    SELECT yourTable.id
    FROM yourTable
    GROUP BY yourGroupingColumn
    ORDER BY yourOrderColumn ASC
    LIMIT 1
  ) AS subquery
)

This query achieves the same result as the previous example by first finding the id of the first row within each group using a subquery, and then selecting the corresponding row from the main table.

Choosing the Right Approach:

• ROW_NUMBER() might be more efficient for large datasets as it avoids a subquery.
• LIMIT with subquery might be easier to understand for beginners.

This example assumes you have a table named orders with columns customer_id, product_id, and price. We want to find the cheapest product (lowest price) for each customer.

SELECT customer_id, product_id, price
FROM orders
GROUP BY customer_id
ORDER BY price ASC  -- Order by price in ascending order (cheapest first)
HAVING ROW_NUMBER() OVER (PARTITION BY customer_id ORDER BY price ASC) = 1;

Explanation:

1. GROUP BY customer_id: This groups all orders by the customer who placed them.
2. ORDER BY price ASC: This orders the rows within each group by price, with the cheapest product first.
3. HAVING ROW_NUMBER()... = 1: This filters the results to only include the row where the ROW_NUMBER() function equals 1. Since the rows are ordered by price with ASC, this ensures we get the cheapest product for each customer.

Example 2: Using LIMIT with Subquery

This example uses the same orders table and retrieves the most expensive product (highest price) for each customer.

SELECT customer_id, product_id, price
FROM orders
WHERE orders.id IN (
  SELECT id
  FROM (
    SELECT orders.id
    FROM orders
    GROUP BY customer_id
    ORDER BY price DESC  -- Order by price in descending order (most expensive first)
    LIMIT 1
  ) AS subquery
)

1. Subquery: This inner query retrieves the id of the most expensive product for each customer.
   • It groups by customer_id.
   • It orders by price with DESC (highest price first).
   • It uses LIMIT 1 to fetch only the first row (most expensive product).
2. Main Query: This outer query selects the entire row from the orders table where the id matches the results from the subquery. This ensures we get the details of the most expensive product for each customer.

1. User-Defined Variables:

This approach involves creating user-defined variables within the query to track the desired row. It can be less efficient for large datasets compared to window functions.

SELECT yourTable.id, yourTable.value
FROM yourTable
ORDER BY yourGroupingColumn, yourOrderColumn ASC;

SET @prev_group = NULL;
SET @selected_id = NULL;

SELECT yourTable.id, yourTable.value
FROM (
  SELECT yourTable.*,
         @prev_group := yourGroupingColumn,
         CASE WHEN @prev_group <> yourGroupingColumn THEN @selected_id := yourTable.id ELSE NULL END AS is_first_row
  FROM yourTable
  ORDER BY yourGroupingColumn, yourOrderColumn ASC
) AS subquery
WHERE is_first_row IS NOT NULL;

• The main query orders the data by yourGroupingColumn and yourOrderColumn (ascending order).
• User variables @prev_group and @selected_id are declared outside the subquery.
• The subquery assigns the current yourGroupingColumn value to @prev_group.
• It checks if the group has changed (yourGroupingColumn is different from @prev_group). If so, it sets @selected_id to the current row's id. Otherwise, it sets it to NULL.
• The final WHERE clause in the main query selects rows where is_first_row (derived from @selected_id) is not null, effectively selecting the first row within each group.

1. CASE Statement with Subquery:

This approach uses a subquery to identify the desired row ID and then uses a CASE statement to filter based on that ID.

SELECT yourTable.id, yourTable.value
FROM yourTable
WHERE yourTable.id IN (
  SELECT id
  FROM (
    SELECT yourTable.id
    FROM yourTable
    GROUP BY yourGroupingColumn
    ORDER BY yourOrderColumn ASC
    LIMIT 1
  ) AS subquery
);

This is similar to the LIMIT with Subquery approach mentioned earlier, but it uses a separate CASE statement within the main query for selection.

Choosing the Right Alternate Method:

• User-defined variables might be a less performant option for large datasets.
• CASE statement with subquery is similar to LIMIT with Subquery but might be less readable for complex scenarios.