How to Resolve the ORA-00979 'Not a GROUP BY Expression' Error in SQL Queries?

ORA-00979: Not a GROUP BY Expression

The ORA-00979 error signifies a mismatch between your SQL query's SELECT list and GROUP BY clause. This typically occurs when a SELECT statement includes columns not present in the GROUP BY clause, and those columns aren't aggregated using functions like SUM, AVG, MIN, MAX, COUNT, etc.

Scenario:

Your query likely involves multiple tables (e.g., review, cs, fact), and the GROUP BY clause doesn't encompass all the non-aggregated columns in the SELECT list. For example:

<code class="language-sql">SELECT cr.review_sk, cr.cs_sk, cf.fact_date, cr.tracking_number,  cf.some_other_column  -- Problem: cf.some_other_column is missing from GROUP BY
FROM review cr
JOIN cs ON ...
JOIN fact cf ON ...
GROUP BY cr.review_sk, cr.cs_sk, cf.fact_date, cr.tracking_number;</code>

Solution:

To fix this, either:

1. Add the missing column(s) to the GROUP BY clause: Include every column from the SELECT list that isn't aggregated in the GROUP BY clause.

2. Aggregate the missing column(s): Use an aggregate function (e.g., SUM(cf.some_other_column)) to summarize the values for the column.

Additional Considerations:

• Non-equality joins: Even if all SELECT columns are in GROUP BY, the error can arise with non-equality joins (e.g., t1.id > t2.id). This creates duplicate rows, violating the GROUP BY principle. Revise your join conditions or use subqueries to eliminate duplicates.

• Understanding GROUP BY: The GROUP BY clause groups rows with identical values in specified columns, allowing aggregate functions to operate on each group. Without proper grouping, the database cannot determine a single value for non-aggregated columns.

By ensuring that every column in your SELECT list is either in the GROUP BY clause or is aggregated, you can resolve the ORA-00979 error and obtain consistent query results.

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