Why Does My SQL Query Throw a 'Column is Invalid in the Select List' Error Related to GROUP BY?

SQL query error: Column is not valid in SELECT list because it does not conform to an aggregate function or GROUP BY clause

When executing a SQL query, you may encounter the following error:

<code>Column 'Column_Name' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.</code>

The reason for this error is as follows:

Data grouping

In the GROUP BY clause, rows with the same column value are specified to be grouped together. For example, in the following query:

<code>SELECT loc.LocationID, emp.EmpID
FROM Employee AS emp FULL JOIN Location AS loc ON emp.LocationID = loc.LocationID
GROUP BY loc.LocationID</code>

Employees are grouped by their LocationID. However, if you try to include a column in the SELECT list that is not in the GROUP BY clause (for example, emp.EmpID), you will get this error because it cannot be determined which EmpID should be selected for each group value.

Aggregation function

Aggregation functions such as SUM, COUNT, MAX, and MIN) can be used to obtain summary information for grouped data. If you include a non-aggregated column in the SELECT list but it is not part of a GROUP BY clause, you must use an aggregate function on it.

For example, if you wanted to calculate the number of employees per location, you would use:

<code>SELECT loc.LocationID, COUNT(emp.EmpID)
FROM Employee AS emp FULL JOIN Location AS loc ON emp.LocationID = loc.LocationID
GROUP BY loc.LocationID</code>

This ensures that for each location, only a count of employees is returned, rather than a list of individual employee IDs.

Solution

To resolve this error you must:

• Add columns to the GROUP BY clause: This will also group the data based on the additional columns.
• Use an aggregate function on the columns in the SELECT list: This will calculate summary information for each group based on the non-aggregated column.

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