Solving the ClickHouse Conundrum: Column xxx is not under aggregate function and not in GROUP BY keys

Are you tired of encountering the infamous ClickHouse error “Column xxx is not under aggregate function and not in GROUP BY keys”? You’re not alone! This error can be frustrating, especially when you’re trying to extract valuable insights from your data. Fear not, dear reader, for we’re about to embark on a journey to conquer this error and unlock the full potential of ClickHouse. Buckle up, and let’s dive in!

Understanding the Error

Before we dive into the solutions, it’s essential to understand why this error occurs in the first place. The error message is quite descriptive, but let’s break it down further:

• Column xxx is not under aggregate function: This part of the error message indicates that the column you’re trying to query is not part of an aggregate function, such as SUM, AVG, or COUNT. ClickHouse requires that columns in the SELECT statement be either aggregate functions or part of the GROUP BY clause.
• and not in GROUP BY keys: This part of the error message suggests that the column is not included in the GROUP BY clause. The GROUP BY clause is used to group rows based on one or more columns. When you include a column in the SELECT statement, it must be part of the GROUP BY clause or an aggregate function.

Solution 1: Add the Column to the GROUP BY Clause

The most straightforward solution is to add the problematic column to the GROUP BY clause. This tells ClickHouse to include the column in the grouping process.

SELECT 
  column1, 
  column2, 
  xxx, 
  COUNT(*) 
FROM 
  table_name 
GROUP BY 
  column1, 
  column2, 
  xxx;

In this example, we’ve added the xxx column to the GROUP BY clause. This ensures that ClickHouse groups the rows based on the values in the xxx column, along with column1 and column2.

Solution 2: Use an Aggregate Function

Another approach is to wrap the problematic column in an aggregate function. This tells ClickHouse to perform a calculation on the column, rather than treating it as a distinct value.

SELECT 
  column1, 
  column2, 
  SUM(xxx), 
  COUNT(*) 
FROM 
  table_name 
GROUP BY 
  column1, 
  column2;

In this example, we’ve wrapped the xxx column in a SUM aggregate function. This calculates the sum of the xxx column for each group, rather than treating it as a distinct value.

Solution 3: Use Any or ArrayAgg

If you’re using ClickHouse 19.3 or later, you can take advantage of the Any or ArrayAgg functions to include the problematic column in your query.

SELECT 
  column1, 
  column2, 
  any(xxx), 
  COUNT(*) 
FROM 
  table_name 
GROUP BY 
  column1, 
  column2;

In this example, we’ve used the Any function to include the xxx column in the query. The Any function returns any value from the group, which can be useful when you’re not concerned with a specific aggregate function.

Alternatively, you can use the ArrayAgg function to collect all values from the group into an array:

SELECT 
  column1, 
  column2, 
  array_agg(xxx), 
  COUNT(*) 
FROM 
  table_name 
GROUP BY 
  column1, 
  column2;

Solution 4: Use a Subquery

In some cases, you may need to use a subquery to extract the desired data. This involves creating a subquery that fetches the required data and then using the results in the main query.

SELECT 
  column1, 
  column2, 
  (
    SELECT 
      xxx 
    FROM 
      table_name 
    WHERE 
      column1 = main.column1 
      AND column2 = main.column2
  ) AS xxx, 
  COUNT(*) 
FROM 
  table_name main 
GROUP BY 
  column1, 
  column2;

In this example, we’ve created a subquery that fetches the xxx column for each group, based on the values in column1 and column2. The subquery is then used in the main query to include the xxx column in the results.

Best Practices for Avoiding the Error

To avoid the “Column xxx is not under aggregate function and not in GROUP BY keys” error, follow these best practices:

1. Always include columns in the GROUP BY clause: When using aggregate functions, ensure that all columns in the SELECT statement are included in the GROUP BY clause.
2. Use aggregate functions wisely: Only use aggregate functions on columns that make sense in the context of your query. Avoid using aggregate functions on columns that are not part of the GROUP BY clause.
3. Avoid selecting unnecessary columns: Only select the columns that are necessary for your query. This reduces the risk of encountering the error and improves query performance.
4. Test your queries thoroughly: Always test your queries with different scenarios to ensure they work as expected. This helps you catch errors early and avoid unexpected results.

Conclusion

In this article, we’ve explored the “Column xxx is not under aggregate function and not in GROUP BY keys” error in ClickHouse and provided four solutions to overcome it. By understanding the error and applying the solutions and best practices outlined above, you’ll be well-equipped to tackle even the most complex ClickHouse queries.

Solution	Description
Add column to GROUP BY clause	Add the problematic column to the GROUP BY clause
Use aggregate function	Wrap the problematic column in an aggregate function, such as SUM or AVG
Use Any or ArrayAgg	Use the Any or ArrayAgg functions to include the problematic column in the query
Use a subquery	Create a subquery to fetch the required data and then use the results in the main query

By following these solutions and best practices, you’ll be able to overcome the “Column xxx is not under aggregate function and not in GROUP BY keys” error and unlock the full potential of ClickHouse. Happy querying!

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