GROUP BY vs. DISTINCT: When Are They Truly Identical in SQL?

SQL's GROUP BY and DISTINCT: When Do They Produce Identical Results?

While both GROUP BY and DISTINCT can yield the same results for a single column in SQL, their underlying mechanisms and intended uses differ. This article explores their similarities and crucial distinctions.

GROUP BY typically works with aggregate functions (like SUM, AVG, COUNT) to group and summarize data. However, when used without aggregates on a single column, it effectively acts like DISTINCT, returning only unique values.

According to MusiGenesis, SQL Server optimizes GROUP BY clauses without aggregates, treating them as DISTINCT and generating a functionally equivalent execution plan. This leads to identical performance in these specific cases.

Nevertheless, Hank highlights a key point: using GROUP BY without aggregation can be misleading. Its primary purpose is data aggregation, and using it to find unique values can cause confusion and potential problems in more complex queries. For retrieving distinct values, explicitly using DISTINCT is best practice – using the right tool for the job prevents unexpected behavior and improves code readability.

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