GROUP BY vs. DISTINCT: When Should You Use Each for Unique Values?

GROUP BY and DISTINCT: detailed comparison

When extracting unique values ​​in a data set, developers often use GROUP BY or DISTINCT. Although these two queries may produce the same results, their underlying processing mechanisms are quite different.

GROUP BY clause is mainly used to aggregate data using summary functions such as SUM, COUNT and AVERAGE. However, when aggregate functions are not used, SQL Server interprets them as DISTINCT operations. In this case, the server optimizes the execution plan to make a single pass through the data, eliminating duplicates.

The DISTINCT clause, on the other hand, is specifically designed to return unique values ​​from a column. It works by comparing the value of each row to the value of every other row, which can be very computationally intensive for large data sets.

So, while GROUP BY (without aggregate functions) and DISTINCT can render the same results, since DISTINCT is more performance-focused, it is still the preferred method for extracting unique values. Additionally, be aware that using GROUP BY for such operations may result in unexpected database behavior. Therefore, it is important to carefully consider the most appropriate tool for a specific task to ensure efficiency and maintain data integrity.

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