When Should You Avoid Using `ORDER BY` in a Subquery?

When Should You Avoid Using Order By in a Subquery?

In SQL, subqueries are often used to provide filtered or aggregated data for use in outer queries. While order by clauses can be applied to subqueries, their use is generally discouraged for several reasons.

One reason to avoid order by in subqueries is its redundancy. Subquery results will be used in the context of the outer query, which will typically have its own order by clause. Ordering the subquery itself adds an unnecessary layer of ordering and can lead to performance issues.

Moreover, order by operations in subqueries may not guarantee the expected order in the final results. SQL query results are inherently unordered unless explicitly ordered by an order by clause. Therefore, using order by in a subquery provides no assurance that the order will be maintained in the outer query.

Furthermore, reliance on order by in subqueries can lead to database-specific implementation issues. Different RDBMSs may handle order by differently, and the results may vary depending on the specific implementation. As a general rule, it is safer to avoid using order by in subqueries.

However, there are exceptions to this rule. If the subquery uses TOP or LIMIT clauses to restrict the number of rows returned, an order by clause is necessary to specify the order in which the rows are retrieved. However, this is not considered standard SQL syntax.

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