Understanding the Problem

MySQL JOINED table query that adds a Count() column returns 1 when zero match

In this article, we’ll delve into the world of SQL and explore how to correctly add a Count() column to a joined table query in MySQL. We’ll examine why the current approach is not yielding the expected results and provide a solution to accurately count the number of comments associated with each group.

The Current Query

The provided SQL query attempts to join three tables: saved_groups, user, and user_comments. It aims to retrieve the date, group name, highlights, display name, and comment count for each saved group. However, it’s encountering an issue when a group has zero comments associated with it.

SELECT 
  sg.date, 
  sg.groupName, 
  sg.highlights, 
  user.display_name as displayName,
  COUNT(*) as cmtcnt 
FROM `saved_groups` as sg 
LEFT JOIN `user` on user.email = sg.userName 
LEFT JOIN `user_comments` as uc on sg.groupName = uc.groupName 
GROUP BY sg.groupName

The Issue

The query returns an incorrect count of 1 for groups with zero comments. This is because the COUNT(*) function in MySQL always returns a value greater than 0, even if there are no rows to count.

-- In SQL Server and PostgreSQL, COUNT() can return NULL 
-- when the subquery returns no rows.

However, in MySQL, COUNT(*) never returns NULL. Instead, it returns the number of non-NULL values in the specified column.

The Misconception

The original query attempts to fix the issue by using various workarounds, such as subtracting 1 from the count or using COALESCE to replace NULL with a specific value. However, these approaches are not accurate and may lead to incorrect results.

-- COALESCE(COUNT(*), 0) will return the first non-NULL value 
-- (in this case, 1), rather than returning 0 for rows with no comments.

The Solution

To accurately count the number of comments associated with each group, we need to specify the groupName column inside the COUNT() function. This ensures that only non-NULL rows are counted.

SELECT 
  sg.date, 
  sg.groupName, 
  sg.highlights, 
  user.display_name as displayName,
  COUNT(uc.groupName) as cmtcnt 
FROM `saved_groups` as sg 
LEFT JOIN `user` on user.email = sg.userName 
LEFT JOIN `user_comments` as uc on sg.groupName = uc.groupName 
GROUP BY sg.groupName

By making this change, we can ensure that the count is accurate and returns 0 for groups with zero comments.

Additional Insights

• NULL vs. 0: In MySQL, it’s essential to understand the difference between NULL and 0. While both values are used to represent a lack of data, they serve distinct purposes in SQL queries.
• COUNT() vs. COUNT(*): The COUNT() function can be used with or without the * wildcard. When using COUNT(*),, MySQL counts all rows, whereas when using COUNT(column_name), MySQL only counts non-NULL values.
• GROUP BY: The GROUP BY clause is used to group rows based on one or more columns. In this example, we’re grouping by the groupName column.

Conclusion

In conclusion, understanding how COUNT() works in MySQL is crucial for writing accurate SQL queries. By specifying the groupName column inside the COUNT() function and avoiding workarounds like COALESCE, we can ensure that our queries return accurate results. Remember to consider the differences between NULL and 0, as well as the nuances of COUNT().

Last modified on 2023-10-05