Why Does My SQL `GROUP BY` Query Without Aggregate Functions Return an Error?

GROUP BY Function without Aggregate Functions

Understanding the GROUP BY operation can be challenging, especially when excluding aggregate functions. This article delves into the concept and its implications.

In the provided example, you attempted to execute a GROUP BY operation on the EMP table without aggregate functions, resulting in several errors. The error message, "not a GROUP BY expression," indicates that the columns specified in the GROUP BY clause must match the columns selected in the SELECT statement.

To comprehend why this is necessary, it's crucial to understand how GROUP BY transforms multiple rows into a single row. When combining rows, it requires guidance for handling columns with varying values. Therefore, every column you include in the SELECT statement must either:

• Be included in the GROUP BY clause
• Be used in an aggregate function

For instance, consider the following table:

Name | OrderNumber
------------------
John | 1
John | 2

If you execute a GROUP BY on Name only, the result requires a rule for selecting the OrderNumber. The options are:

• Include OrderNumber in the GROUP BY clause: This produces two rows:

  • John | 1
  • John | 2

• Use an aggregate function:

  • MAX(OrderNumber): Result: John | 2
  • SUM(OrderNumber): Result: John | 3

By matching the columns in the SELECT and GROUP BY statements, Oracle ensures consistent data manipulation and avoids ambiguity.

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