Does select distinct affect performance?
Yes, basically it has to sort the results and then re-processed to eliminate the duplicates. This cull could also be being done during the sort, but we can only speculate as to how exactly the code works in the background. You could try and improve the performance by creating an index composed of all three (3) fields.
Can distinct be used in WHERE clause?
Within the WHERE clause lies many possibilities for modifying your SQL statement. Among these possibilities are the EXISTS, UNIQUE, DISTINCT, and OVERLAPS predicates. Here are some examples of how to use these in your SQL statements.
Does WHERE clause improve performance?
A where clause will generally increase the performance of the database.
- Generally, it is more expensive to return data and filter in the application.
- The database can optimize the query, using indexes and partitions.
- The database may be running in parallel, executing the query in parallel.
Can we use distinct in subquery?
The DISTINCT keyword cannot be used with subqueries that include `GROUP BY.
How to select distinct and top in SQL?
FROM (SELECT distinct personID FROM TableA) t 2: ;with cte as (SELECT distinct personID FROM TableA) SELECT top 10 personID FROM cte 3: ;with cte as (SELECT personID,row_count=row_number () over (partition by personID order by personID) FROM TableA ) SELECT top 10 personID FROM cte where row_count=1
How to make select distinct with where clause?
I want to write SQL querye returning all persons, that have not kvalifikation 2. But this return both person 1 and person 2. How do i make select that only return personId’s that not have kval2 ?
How are distinct and top expressions treated the same?
Expressions are treated the same as column regarding DISTINCT and TOP. Let’s start with a select statement to get the first name as well as the full, which we create by appending LastName to FirstName. Also, keep in mind, when using ORDER BY, that the ORDER BY items must appear in the select list when using Distinct.
Is it good to use distinct keyword in PLSQL?
Yes, as using DISTINCT will (sometimes according to a comment) cause results to be ordered. Sorting hundreds of records takes time. Try GROUP BY all your columns, it can sometimes lead the query optimiser to choose a more efficient algorithm (at least with Oracle I noticed significant performance gain).