contrast: I know it's an old post. Difference between GROUP BY and ORDER BY in Simple Words Hello Pinal, I never opine but I constantly read your posts. Performance Surprises and Assumptions : GROUP BY vs. DISTINCT Connect and share knowledge within a single location that is structured and easy to search. Difference between distinct and group by in SQL Server If you want to group your results, use GROUP BY, if you just want a unique list of a specific column, use DISTINCT. Thanks! In New column name, enter Total units, in Operation, select Sum, and in Column, select Units. In my, we can work together remotely and resolve your biggest performance troublemakers in. The query plans showed that: in DISTINCT, my static column like : < "TagString" as tag > will be included into group-keys, but not when using 'group by key'. Comment * document.getElementById("comment").setAttribute( "id", "a748006fd1a4044a66646e0de4daa7b2" );document.getElementById("da608376e8").setAttribute( "id", "comment" ); Save my name, email, and website in this browser for the next time I comment. But I get the idea that it is redundant to use both in almost all cases! Have you ever opened any PowerPoint deck when you face SQL Server Performance Tuning emergencies? DISTINCT. Overview In this article, we'll discuss distinct HQL queries and how to avoid adding the distinct keyword in SQL queries when it's not necessary. Essentially I share my business secrets to optimize SQL Server performance. I personally prefer the distinct syntax, but I am sure it's more out of habit than anything else. How to launch a Manipulate (or a function that uses Manipulate) via a Button. is an old syntax which was used in oracle description but later ANSI standard defines DISTINCT as the official keyword. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Consider the following table: The query below uses GROUP BY to perform aggregated calculations. The interesting thing is, for simple case like above, the query plans are the same (all using group-by) but different when I put many DISTINCT+UNION versus GROUPBY+UNION. The difference here since we have to guess (since we don't have the explain plans) is IMO that the inline subquery gets executed AFTER the GROUP BY but BEFORE the DISTINCT. If all you need is to remove duplicates, then use DISTINCT. PARTITION BY gives you more flexibility in choosing the grouping columns. To freely share his knowledge and help others build their expertise, Pinal has also written more than 5,600 database tech articles on his blog at https://blog.sqlauthority.com. Thank you very much ! DISTINCT -> no or a few duplicates only . Hi. Interview Question of the Week #034 - What is the Difference Between The By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. In SQL Server you can get Query Execution Plans.. can you get something similar in Oracle? SQL Tutorial => Difference between GROUP BY and DISTINCT Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing, This is not a question about aggregates, it is a GROUP BY functioning the same as a distinct when no aggregate function is present, One very minor difference that I haven't seen mentioned is that. SQL Server Education (by the geeks, for the geeks). Unfortunately, I have to live with what I have. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. Nupur Dave is a social media enthusiast and an independent consultant. When we migrated from Oracle 9i to 11g the response time in Toad was excellent but in the reporte it took about 35 minutes to finish the report when using previous version it took about 5 minutes. Connect and share knowledge within a single location that is structured and easy to search. You could post this on Hadoop/Hive's issue tracker or something, but you'll still probably just have to roll with it. I imagine that as Hive matures, such problems will be fixed. Is SELECT DISTINCT always redundant when using a GROUP BY clause? Interview Question of the Week #020 - What is the Difference Between 2. You are definitely right on this point. By clicking Post Your Answer, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct. Stick with us as we discuss and demo the differences between SQL DISTINCT vs UNIQUE. no .group by is faster! Was Hunter Biden's legal team legally required to publicly disclose his proposed plea agreement? with w as (select round(level/2) as id from dual connect by level < 11). Is your SQL Server running slow and you want to speed it up without sharing server credentials? It may be differ vendor wise. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. Browse other questions tagged, Where developers & technologists share private knowledge with coworkers, Reach developers & technologists worldwide, The future of collective knowledge sharing. What can I do about a fellow player who forgets his class features and metagames? If you look at the exec plan (ok, it may depend on your database) the DISTINCT cost adds on the analytical query, which is already a twice as costly as the simple group-by. Apart from the fact that unlike DISTINCT, GROUP BY allows for aggregating data per group (which has been mentioned by many other answers), the most important difference in my opinion is the fact that the two operations "happen" at two very different steps in the logical order of operations that are executed in a SELECT statement. ORA-00937: - "not a single-group group function". What I basically meant was from what I have read and understand is that using. Dunno if you can count on that in all situations, though. Funtional efficiency is totally different. I have not seen the single reducer effect for distinct versus group by. On your point 2. some (one?) GROUP BY should be used to apply aggregate operators to each group. The functional difference is thus obvious. With GROUP BY you can have only one set of grouping columns for all aggregated columns. What I need to do for this? DISTINCT. Quantifier complexity of the definition of continuity of functions, Simple vocabulary trainer based on flashcards. Note also that 4 tables are not used in your original select. Postgresql : which way is faster DISTINCT or GROUP BY? Make a decision of which one is better in your query by checking the execution plans and determine the relative efficiency of queries that generate the same result set. When it comes to SQL you always have both a screwdriver and hammer available. Pinal Dave, thank you for your postings; you have been extremely helpful. The query given is just an example that shows one of the distantly joined tables being used to filter results. http://sqlmag.com/database-performance-tuning/distinct-vs-group. +1 for code smell. MySQL - What is the difference between GROUP BY and DISTINCT? We'll use the Post and Comment entity objects, which share a one-to-many relationship. Why such a huge disparity between execution times? Constraints In real-life scenarios, there always has been a need for constraints on data so that we may have data that is mostly bug-free and consistent to ensure data integrity. Solution for speeding up a slow SELECT DISTINCT query in Postgres. You should still use SEMI-JOIN (EXISTS or IN) when appropriate instead of DISTINCT, it is clearer to both future reader and perhaps more importantly to the optimizer. In oracle there are lot more analytic function than aggregation function. Making statements based on opinion; back them up with references or personal experience. Note: In the following query, WHERE TASK_INVENTORY_STEP.STEP_TYPE = 'TYPE A' represents just one of a number of ways that results can be filtered. SQL Group By vs Distinct Difference between GROUP BY and DISTINCT GROUP BY is used in combination with aggregation functions. group by already filter out duplicated row. SQL SERVER SPID is KILLED/ROLLBACK state. In this article, we'll demonstrate how you can use the GROUP BY clause in practice. Did Kyle Reese and the Terminator use the same time machine? AVG, MAX, MIN, SUM, and COUNT on Specific column and fetch You remind me of Third-Normal-Form. I think ms-sql is the same case. Semantic search without the napalm grandma exploit (Ep. A DISTINCT and GROUP BY usually generate the same query plan, so performance should be the same across both query constructs. It is too obvious but I didn't even realize it! Understanding the Problem First, let's look at our data model and identify what we're trying to accomplish. Sql DISTINCT vs GROUPBY Clause - simmanchith Select OK. Are these two queries the same - GROUP BY vs. Example # GROUP BY is used in combination with aggregation functions. I'm fairly sure that GROUP BY and DISTINCT have roughly the same execution plan. DISTINCT is just a hack. She primarily focuses on the database domain, helping clients build short and long term multi-channel campaigns to drive leads for their sales pipeline. Because GROUP BY implicitly does a DISTINCT over the values of the column you're grouping by (sorry for the cacophony). you don't understand why "b=b" would return all rows in your case? Was there a supernatural reason Dracula required a ship to reach England in Stoker? The way I always understood it is that using distinct is the same as grouping by every field you selected in the order you selected them. - jarlh Jul 5, 2017 at 14:57 To subscribe to this RSS feed, copy and paste this URL into your RSS reader. When you have a result set containing more than one duplicate records, then you can get unique results out of that by using DISTINCT. 5 Examples of GROUP BY | LearnSQL.com Find centralized, trusted content and collaborate around the technologies you use most. Pinal is an experienced and dedicated professional with a deep commitment to flawless customer service. how is this an answer? Two things; 1) Put your GROUP BY query in your question and 2) Run an EXPLAIN PLAN on each query and also add the output to the question. How do I know how big my duty-free allowance is when returning to the USA as a citizen? not. And of course, keep up to date with AskTOM via the official twitter account. How to Use GROUP BY and HAVING in SQL | DataCamp How to launch a Manipulate (or a function that uses Manipulate) via a Button. Was Hunter Biden's legal team legally required to publicly disclose his proposed plea agreement? summarize operator - Azure Data Explorer | Microsoft Learn Difference between SQL Distinct and Group By. In Hive (HQL), GROUP BY can be way faster than DISTINCT, because the former does not require comparing all fields in the table. I have seen this both in my experience, and it is documented and discussed (for example, on slides 26 and 27 in this presentation). example. 11 I have found some SQL queries in an application I am examining like this: SELECT DISTINCT Company, Warehouse, Item, SUM (quantity) OVER (PARTITION BY Company, Warehouse, Item) AS stock I'm quite sure this gives the same result as: SELECT Company, Warehouse, Item, SUM (quantity) AS stock GROUP BY Company, Warehouse, Item What are the long metal things in stores that hold products that hang from them? Why does a flat plate create less lift than an airfoil at the same AoA? Here is the Oracle query plan for the query using DISTINCT: Here is the Oracle query plan for the query using GROUP BY: The performance difference is probably due to the execution of the subquery in the SELECT clause. Group By is intended to be used like this: Which would show the sum of all transactions for each person. Data groupingor data aggregationis an important concept in the world of databases. Which is more efficient, distinct or group by in MySQL? - SoByte The "GROUP BY" clause is used when you need to group the data and it should be used to apply aggregate operators to each group. You should use GROUP BY to apply aggregate operators to each group and DISTINCT if you only need to remove duplicates. (Some aggregation functions return multiple columns.) But I think that because the operation names are different, the execution would follow somewhat different code paths and that opens the possibility of more significant differences. Running fiber and rj45 through wall plate. Is SELECT DISTINCT always redundant when using a GROUP BY clause? Consider the following table: The query below uses GROUP BY to perform aggregated calculations. Then the specified aggregation functions are computed over each group, producing a row for each group. Find more tutorials on the SAS Users YouTube channel. Create a grouping using all of the GroupBy columns (which are required to exist in the table from step #1.). is my MOST popular training with no PowerPoint presentations and, Comprehensive Database Performance Health Check, SQL SERVER Difference between DISTINCT and GROUP BY Distinct vs Group By. What are the long metal things in stores that hold products that hang from them? How to make a vessel appear half filled with stones. First, we need to look deeper into that question. sql - distinct vs group by which is better - Stack Overflow I take same task and analyze by postgres commands. How to use DISTINCT when I have multiple column in SQL Server? The use case would be for when a single grouping would not suffice all of the aggregates needed. Your contributions to the SQLers are priceless, I just wanted to take the time to say thank you! Community initiative by, sys.dm_exec_describe_first_result_set Day 46 One DMV a Day. SQL Performance: SELECT DISTINCT versus GROUP BY, Distinct and Group By - query performance. So what you want to do is query against this base materialized view, which can be refreshed constantly on the back-end, the persistence strategy involved should not choke out the materialized view (persisting a few hundred records at a time won't crush anything). But use it with care A simple rule of thumb could be: if you can compute it using a group-by, well, don't use an analytical function ;). Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. please ask questions in ONE and ONLY ONE place. The application executes several large queries, such as the one below, which can take over an hour to run. Thus, to conclude there is a functional difference as mentioned above even if the group by produces same result as of distinct. In sql server 2005, it looks like the query optimizer is able to optimize away the difference in the simplistic examples I ran. Pinal is also a CrossFit Level 1 Trainer (CF-L1) and CrossFit Level 2 Trainer (CF-L2). Like us on FaceBook | Join the fastest growing SQL Server group on FaceBook, Your email address will not be published. But why complicate things when SELECT DISTINCT is so easy. Can 'superiore' mean 'previous years' (plural)? That's the point! 2. as we know, they generate same query plan which had been repeatedly mentioned in some items like Which is better: Distinct or Group By. I read all the above comments but didn't see anyone pointed to the main difference between Group By and Distinct apart from the aggregation bit. Suppose, we need information like product name, price, total available stock quantity, and total available stock Rs. If I want to do calculations like summing up the total quantity of mangoes, I will use GROUP BY. Why is groupBy() a lot faster than distinct() in pyspark? on the Microsoft Azure Marketplace. Look in the other place you asked (and I answered) this same exact question. other worse thing we saw is the function, RAM eating. Was Hunter Biden's legal team legally required to publicly disclose his proposed plea agreement? I couldn't find that article again. EDIT: This is not a question about aggregates. Site design / logo 2023 Stack Exchange Inc; user contributions licensed under CC BY-SA. +1 - This is exactly what I was thinking of too (including the potential solution), but I don't know enough about Oracle to be sure. DISTINCT operates not only on a single column of a table but also has support for multiple columns of a table, where DISTINCT in SQL will eliminate those rows where all the selected columns are identical. Very enlightening, thank you. rev2023.8.21.43589. Replacing the DISTINCT with a GROUP BY clause in the query below shrank execution time from 100 minutes to 10 seconds. I was quite surprised when I came across this functionality. I get: For group by query explain (analyze) select product_id, size from logistic.product_stock where status = 'STOCK' group by product_id, size I get next: As we can see: steps are same in both situations. So depending on your use case it's worth to optimize a query by your expensive developer. Hi there. I am using vb6 & ms sql server2000. GROUP BY lets you use aggregate functions, like AVG, MAX, MIN, SUM, and COUNT. I have two tables. In the case of DISTINCT, the rows are redistributed immediately without any preaggregation taking place, while in the case of GROUP BY, in a first step a preaggregation is done and only then are the unique values redistributed across the AMPs. sql - DISTINCT with PARTITION BY vs. GROUPBY - Stack Overflow pinal @ SQLAuthority.com, SQL SERVER Remove All Characters From a String Using T-SQL. The DISTINCT operation "happens after" the projection, so we can no longer remove DISTINCT ratings because the window function was already calculated and projected. Question: What is the difference between DISTINCT and GROUP BY? What is the meaning of tron in jumbotron? Making statements based on opinion; back them up with references or personal experience. The use of GROUP BY with aggregate functions is understood. Is there a RAW monster that can create large quantities of water without magic? As you can see, the logical order of each operation influences what can be done with it and how it influences subsequent operations. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. Usually, developer time is a fixed cost while computer time is incurred every time you run the query. Dont think now that GROUP BY is always better from a performance point of view. See: https://sqlperformance.com/2017/01/t-sql-queries/surprises-assumptions-group-by-distinct. SQL: difference between PARTITION BY and GROUP BY using AI from SAS sign in with your SAS profile. I checked the execution plans for two functionally equivalent queries along these lines in Oracle 10g: The middle operation is slightly different: "HASH GROUP BY" vs. "HASH UNIQUE", but the estimated costs etc. When these two behave similarly and when differently. Or if video is more your thing, check out Connor's latest video and Chris's latest video from their Youtube channels. Pinal Dave is an SQL Server Performance Tuning Expert and independent consultant with over 21 years of hands-on experience. Again, we are taking same as above table and getting unique records from the result set by using GROUP BY clause. By clicking Accept all cookies, you agree Stack Exchange can store cookies on your device and disclose information in accordance with our Cookie Policy. The two queries return same result. Not the answer you're looking for? It isn't something I plan to use, but a way things have been done at this new place I am working at. Not the answer you're looking for? They have different semantics, even if they happen to have equivalent results on your particular data. Is there any difference to use group by in a query ? GROUP BY should be used to apply aggregate operators to each group. DISTINCT is a filter that separates unique records from those that meet the query requirements. Thanks for reading. Behavior of narrow straits between oceans. Select Group by on the Home tab. select deptno,min (sal),max (sal),sum (sal) from emp group by deptno; it u may try u can understand. What is the best way to say "a large number of [noun]" in German? In order to use DISTINCT, we'd have to nest that part of the query: Side-note: In this particular case, we could also use DENSE_RANK(). Asking for help, clarification, or responding to other answers. Making statements based on opinion; back them up with references or personal experience. Connect and share knowledge within a single location that is structured and easy to search. SQL Group by clause. SQL - SQL Group By vs Distinct - DevTut When the performance of Distinct and Group By are different? are identical. GROUP BY can (again, in some cases) filter out the duplicate rows before performing any of that work. Usually, if the record counts are different, there is something I hadn't considered. MusiGenesis' response is functionally the correct one with regard to your question as stated; the SQL Server is smart enough to realize that if you are using "Group By" and not using any aggregate functions, then what you actually mean is "Distinct" - and therefore it generates an execution plan as if you'd simply used "Distinct.". She primarily focuses on the database domain, helping clients build short and long term multi-channel campaigns to drive leads for their sales pipeline. If all you need is to remove duplicates then use DISTINCT. No, the distinct will be in general much worse - the optimizer recognizes top-n quereis with row_number(). Essentially, DISTINCT collects all of the rows, including any expressions that need to be evaluated, and then tosses out duplicates.

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