How Does GROUP BY Work Without Aggregate Functions?

GROUP BY Explained Without Aggregate Functions

When using the GROUP BY clause without aggregate functions, it's essential to understand the concept of collapsing rows into a single row. In this case, the clause operates by combining multiple rows with identical values for specific fields into one row. This process, however, raises the question of how conflicting values in other fields should be handled.

As demonstrated in the examples you provided, selectively including or excluding fields in the GROUP BY clause yields either valid or invalid results. Valid results are generated when the number of columns in the GROUP BY clause is equal to the number of columns selected in the SELECT statement.

For instance, in the query "SELECT ename, sal FROM emp GROUP BY ename, sal," the presence of both "ename" and "sal" in both the SELECT and GROUP BY clauses returns the intended output.

Invalid results arise when the number of columns in the GROUP BY clause does not match the number selected in the SELECT statement. In the queries "SELECT ename, sal FROM emp GROUP BY ename;" and "SELECT ename, sal FROM emp GROUP BY sal;," the absence of a grouping expression for one of the selected columns triggers the error message "not a GROUP BY expression."

To avoid such errors, remember that when using GROUP BY without aggregate functions, the number of columns in the GROUP BY clause must always match the number of columns selected in the SELECT statement. This ensures that the system has clear instructions on how to combine duplicate values and produce a valid result.

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