MySQL Query to Get Auction Data and Bids from Two Tables Using JOIN

In this article, we will explore how to retrieve data from two tables using a JOIN clause in MySQL. We will use an example of getting auction data and bids from two tables a and b. The goal is to display the amount of bids, total sum of bids, last bid date per auction, along with their IDs and statuses from table a, ordered by status.

Understanding Table Structure

Before we dive into the query, let’s understand the structure of our tables. We have two tables: auctions (table a) and bids.

Table a (Auctions)
+----+--------+
| id | status |
+----+--------+
| 1  | 1      |
| 2  | 0      |
| 3  | 0      |
| 4  | 1      |
+----+--------+

Table b (Bids)
+----+-----------+-------+------------+
| id | auction_id | amount | date       |
+----+-----------+-------+------------+
| 1  | 1          | 0.1   | 2018-01-24 |
| 2  | 1          | 0.1   | 2018-01-24 |
| 3  | 4          | 0.2   | 2018-01-26 |
| 4  | 4          | 0.1   | 2018-01-26 |
| 5  | 4          | 0.3   | 2018-01-28 |
| 6  | 1          | 0.1   | 2018-01-29 |
+----+-----------+-------+------------+

Current Query Issue

The given query is:

SELECT 
    a.id, 
    a.status, 
    SUM(b.amount) as bids_sum, 
    COUNT(b.id) as bids_count, 
    MAX(b.dt) as last_bid_date 
FROM a left outer join b 
  on a.id = b.auction_id 
GROUP BY a.id 
ORDER BY a.status DESC, a.id DESC

This query returns only the auctions that have at least one bid. The problem is to retrieve all auctions without excluding any.

Solution: Using LEFT OUTER JOIN and COALESCE

To solve this issue, we can use a LEFT OUTER JOIN instead of an INNER JOIN. This will ensure that we include all rows from table a, even if there are no matching rows in table b.

SELECT 
    a.id, 
    a.status, 
    COALESCE(SUM(b.amount), 0) as bids_sum, 
    COUNT(b.id) as bids_count, 
    COALESCE(MAX(b.dt), '0000-00-00') as last_bid_date 
FROM a left outer join b 
  on a.id = b.auction_id 
GROUP BY a.id 
ORDER BY a.status DESC, a.id DESC

In this query:

• We use LEFT OUTER JOIN instead of INNER JOIN to include all rows from table a.
• The COALESCE function is used to handle cases where there are no bids for an auction. If the sum of bids or last bid date is NULL, it returns 0.

Explanation

Let’s break down what each part of the query does:

SELECT Clause

SELECT 
    a.id, 
    a.status, 
    COALESCE(SUM(b.amount), 0) as bids_sum, 
    COUNT(b.id) as bids_count, 
    COALESCE(MAX(b.dt), '0000-00-00') as last_bid_date

• SELECT clause selects the columns we want to retrieve.
• We use COALESCE to handle cases where there are no bids or last bid date. If the sum of bids, count of bids, or max date is NULL, it returns 0.

FROM Clause

FROM a left outer join b 
  on a.id = b.auction_id

• The FROM clause specifies the tables we want to retrieve data from.
• We use LEFT OUTER JOIN to include all rows from table a, even if there are no matching rows in table b.

GROUP BY Clause

GROUP BY a.id 

• The GROUP BY clause groups the result set by the selected columns.
• In this case, we group the results by the auction ID (a.id) so that we can calculate the sum of bids and count of bids for each auction.

ORDER BY Clause

ORDER BY a.status DESC, a.id DESC

• The ORDER BY clause sorts the result set in ascending or descending order based on the specified columns.
• In this case, we sort the results by the status (a.status) in descending order and then by the auction ID (a.id) in descending order.

Example Output

Here’s an example of what the output might look like:

In this example, the first two rows represent auctions with bids (1 and 4), the third row represents an auction without any bids (3), and the last two rows have null values for sum of bids or count of bids because there are no bids.

By using LEFT OUTER JOIN and COALESCE, we can retrieve all auctions, including those without any bids, while still displaying meaningful data.

Last modified on 2024-04-08