Can we use ORDER BY in subquery in SQL Server query?
Can we use ORDER BY in subquery in SQL?
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18-Jan-2024
Updated on 27-Nov-2024
Tanisha Soni
27-Nov-2024For Subqueries in SQL, Can We Use ORDER BY?
The ORDER BY clause, however, can be used in subqueries in SQL, but allow this usage depends on a few certain conditions. Below is a detailed explanation of scenarios where ORDER BY can or cannot be used in subqueries:
1. When ORDER BY is Valid in Subqueries
a. Subqueries with TOP or LIMIT
Another typical approach to using ordering in subqueries is the attempt to limit the count of rows in the result of the subquery. For example, using TOP (in case with SQL Server) or LIMIT (in case with MySQL, PostgreSQL, or SQLite) ORDER BY guarantees that the correct rows only are filtered in the specified order.
Example: In order to query the 3 first highest valued salaries for employees, you can use ORDER BY and LIMIT or TOP and subqueries.
b. Subqueries Used in SELECT Clause
In the case where a subquery appears in the SELECT clause to select a single scalar value like MAX, MIN, ORDER BY is allowed provided it is combined with LIMIT or TOP. But the ORDER BY clause alone is meaningless with reference to single-value subqueries, unless it decides which row will be used to arrive at the value.
c. Subqueries in the FROM Clause
In other words, if a subquery contains ORDER BY in its FROM clause, the return values are considered to be a derived table. That being said, the order is not guaranteed in the execution of the outer query unless the outer query is ordered using the ORDER BY clause as well.
2. Limitations that apply to the use of the ORDER BY clause in subqueries
a. SUBQUERIES IN IN, EXISTS OR JOIN
Whenever a subquery is used in a condition like IN or EXISTS, the SQL engine most often than not does not consider the ORDER BY clause. This is because the order of the rows in these contexts does not play any role with the operation.
b. Without LIMIT or TOP
ORDER BY without a limiting clause (such as LIMIT or, in TSQL or Sybase, TOP) in subqueries typically results in an error or is omitted. For example, in MySQL, some of such usage can lead to syntax errors: On the other hand, some other databases can just ignore the order.
3. Using Nested Queries
One type is to utilize ORDER BY in a subquery to sort rows and then employ the outer query to select some of the rows or filter out some of them. The outer query itself also has its own ORDER BY to specify the order by which ointment should appear in the output.
4. Performance Considerations
While using ORDER BY when dealing with subqueries, particularly with large tables, a given operation may slow down due to the execution of the sorting process. This problem can be somewhat relieved by indexing records in the columns that are to be ordered in the ORDER BY clause.