Query Execution in SQL: Aggregator Order with Examples

In SQL (Structured Query Language), queries often involve aggregations (functions like SUM, COUNT, AVG, etc.) to summarize data. These aggregations are calculated within groups defined by GROUP BY clauses. However, when multiple aggregators appear in a query, the order in which they are evaluated might be crucial for the final result.

Aggregator Execution Order:

While the specific execution order may vary slightly depending on the SQL database system you're using, here's a general guideline:

1. Filtering (WHERE Clause): The WHERE clause filters rows based on specified conditions before any aggregations are performed. Only rows that meet the filtering criteria are considered for further processing.
2. JOIN Operations: If the query involves JOIN operations to combine data from multiple tables, these joins are executed first to create a single result set.
3. Grouping (GROUP BY Clause): Rows in the result set are then grouped based on the expressions specified in the GROUP BY clause. This partitions the data into groups for subsequent aggregations.
4. Non-Aggregator Functions: Any non-aggregation functions applied within the SELECT clause (e.g., CASE statements, mathematical operations involving individual column values) are evaluated at this stage, once the groups are formed.
5. Aggregation: Finally, the aggregators (e.g., SUM, COUNT, AVG) are calculated within each group. The order in which aggregators are evaluated within a group is generally not guaranteed to be deterministic (meaning it might not always be the same across different systems or query optimizations). However, the order typically won't affect the final result, as these functions operate independently on each group.

SELECT city, COUNT(*), SUM(population)
FROM customers
GROUP BY city;        

In this query:

• COUNT(*) counts the number of customers in each city.
• SUM(population) calculates the total population across customers in each city (assuming there's a population column).

The order in which COUNT(*) and SUM(population) are evaluated doesn't affect the final result because they operate independently on each city group. You'll get the same total count and population sum for each city regardless of the order.

Example 2: Order Can Affect Results (Rare Cases):

One rare scenario where the order of aggregators might influence the outcome is when using window functions (functions that operate on a set of rows within a partition) within the aggregations themselves. However, such cases are uncommon and involve specific window functions that rely on previously calculated aggregations within the same window.

General Recommendation:

Unless you're dealing with very specific window functions, the order of aggregators in most SQL queries won't affect the final result. However, if you're working with a complex query or have concerns, it's always a good practice to check your database system's documentation for any specific order-related considerations.

Key Points:

• Filtering (WHERE) happens before aggregations.
• JOIN operations are executed before aggregations.
• Grouping (GROUP BY) defines groups for aggregations.
• Non-aggregator functions are evaluated after grouping.
• Aggregator order within a group is usually not deterministic but often doesn't affect the result.
• In rare cases, window functions can be affected by aggregator order.

I hope this comprehensive explanation clarifies the query execution order in SQL, particularly regarding aggregators!