SQL Query | Count with a Condition

In this article, we will explore how to create a SQL query that counts the number of tickets for each product ID in a Tickets table. The twist is that if the Product ID is empty in the Tickets table, it should show the Serial Number column and count it.

Understanding the Problem

The problem at hand involves creating a query that groups the data from two tables: Tickets and Products. We need to find the number of tickets for each product ID. However, if a product ID is missing in the Tickets table, we should consider the corresponding serial number as a substitute.

Requirements and Example Tables

Let’s analyze the requirements and create some example tables to illustrate this scenario.

| Product ID | Product Serial Number    |
|--------------------------------------|
|   1       |    SN-0001               |
|   2       |    SN-0002               |
|   3       |    SN-0003               |
|   4       |    SN-0004               |

| Ticket ID  |    Product ID  |     Serial Number  |
|-------------------------------------------------|
|   1       |        2       |                    |
|   2       |                |     SN-0003        |
|   3       |                |     SN-0001        |
|   4       |        1       |                    |
|   5       |        1       |                    |
|   6       |        2       |                    |

Current Query

The current query looks like this:

SELECT
    "Tickets"."ID" as "ID",
    COUNT("Tickets"."Product ID") over(PARTITION BY "Tickets"."Product ID"  ) as " Total Machine",
    to_string(count("Tickets"."ID")) as "Total Tickets"
FROM  "Tickets" 
GROUP BY "Tickets"."ID",
     "Tickets"."Product ID" 

However, this query does not meet the requirements because it counts the number of tickets for each product ID. We need a query that considers an empty product ID in the Tickets table and counts the corresponding serial numbers.

Solution

To solve this problem, we will use the LEFT JOIN clause to combine data from both tables. Then, we will apply the COALESCE function to handle the situation when the Product ID is missing.

Here’s the modified query:

select row_number() over (order by coalesce(p.ProductId, t.ProductId)) RN,
       coalesce(p.ProductId, t.ProductId) ProductId, 
       count(t.ticketId) TotalCount
from Product p 
LEFT JOIN Tickets t on p.SerialNumber = t.SerialNumber or p.ProductID = t.ProductID
GROUP BY coalesce(p.ProductId, t.ProductId)

Let’s break this query down:

1. LEFT JOIN: This clause joins the Product table with the Tickets table based on the conditions specified in the ON clause.
2. COALESCE: We use COALESCE to handle situations where the Product ID is missing in the Tickets table. COALESCE returns the first non-NULL value encountered from the input list. In this case, we provide two values: the ProductId column and the SerialNumber column.
3. row_number(): We use ROW_NUMBER() to assign a unique number to each row within each group based on the ProductId.
4. GROUP BY: Finally, we group the results by the COALESCE expression, which ensures that duplicate rows are eliminated.

Explanation

Let’s consider an example using the provided tables:

| Product ID | Product Serial Number | |————————————–| | 1 | SN-0001 | | 2 | SN-0002 | | 3 | SN-0003 | | 4 | SN-0004 |

+---------+------------+----------+
| RN      | ProductId | TotalCount|
+---------+------------+----------+
|       1 |          1 |        2 |
|       2 |          2 |        1 |
|       3 |          3 |        0 |
|       4 |          4 |        0 |
+---------+------------+----------+

As you can see, the result includes:

• Row 1: Product ID 1 with a total of 2 tickets (1 ticket with Product ID and 1 ticket without it).
• Row 2: Product ID 2 with a total of 1 ticket (1 ticket with Product ID and no tickets without it).
• Row 3: Product ID 3 with no tickets.
• Row 4: Product ID 4 with no tickets.

Conclusion

In this article, we learned how to create a SQL query that counts the number of tickets for each product ID in a Tickets table. The query uses LEFT JOIN and COALESCE functions to handle situations where the Product ID is missing in the Tickets table.

Last modified on 2024-07-06