SELECT DISTINCT & group by diff

原创已于 2024-03-28 17:52:56 修改 · 536 阅读

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于 2024-03-28 17:52:30 首次发布

本文探讨如何避免使用SELECT DISTINCT来提高SQL查询性能，通过GROUP BY实现相同功能。举例说明了如何用GROUP BY代替SELECT DISTINCT从表中获取唯一类别，并提醒在使用GROUP BY时需注意的潜在差异和结果一致性问题。

To avoid using SELECT DISTINCT, you can often rephrase the query using other techniques to achieve the same result without the performance overhead of removing duplicates. Here's an example of how to avoid using SELECT DISTINCT:

Consider a table named products with columns category and price. Let's say you want to retrieve a list of unique categories from the table.

Original query using SELECT DISTINCT:

SELECT DISTINCT category FROM products;

Alternative approach using GROUP BY:

SELECT category FROM products GROUP BY category;

In this alternative approach, the GROUP BY clause achieves the same result as SELECT DISTINCT by grouping the rows based on the category column. This avoids the need for MySQL to sort and eliminate duplicates explicitly, potentially resulting in better performance.

However, it's essential to be cautious when using GROUP BY, especially if you're retrieving additional columns alongside the grouped column. In such cases, the GROUP BY behavior might differ from the DISTINCT behavior, leading to unexpected results.

So, whenever possible, aim to rewrite queries to eliminate the need for SELECT DISTINCT by using other SQL constructs like GROUP BY or by restructuring the query logic to ensure unique results.

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