replace group by with subquery

Stack Overflow for Teams is moving to its own domain! This is a repost of my question on Stack Overflow. '||fl.segment5 location, left join fa_books b on b.asset_id = p.asset_id, group by p.asset_id, p.asset_cost_acct, p.location, p.account_description. It this article, I will discuss the benefits and the drawbacks of each approach. Browse other questions tagged, Start here for a quick overview of the site, Detailed answers to any questions you might have, Discuss the workings and policies of this site, Learn more about Stack Overflow the company, what do you mean by 'eliminate the rows from the subquery (like a where)' ? From this article, the author decides to use WITH to replace subqueries used of this manner: SELECT c.CategoryName, p.ProductName, p.UnitPrice FROM Categories c INNER JOIN (SELECT CategoryId, MAX (UnitPrice) AS MaxPrice FROM Products GROUP BY CategoryId) maxprice ON maxprice.CategoryId = c.CategoryId INNER JOIN Products p ON p.CategoryId = c.CategoryId AND p.UnitPrice = maxprice.MaxPrice ORDER BY MaxPrice DESC. Close the login form. From this article, the author decides to use WITH to replace subqueries used of this manner: Now this article is over 6 years old but it is still relevant to me because I am using SQL Server 2008. Table creation details. It is common to write the queries using GROUP BY and HAVING clause to group records or rows. In this article, I provide five subquery examples demonstrating how to use scalar, multirow, and correlated subqueries in the WHERE, FROM/JOIN, and SELECT clauses. The GROUP BY is an optional clause of the SELECT statement. Usually, this means ordering by RAND() to show, say, 10 random users. SQL and Hive GROUP BY Alternative-Example. the above query gives me latest record as i am trying to find max date based on SYDATE. But there is a third clause here, ORDER BY. These fields will return an arbitrary value from any of the aggregated records (in practice, that is the record first read in its group). All other values from users are uniquely defined by the PRIMARY KEY so there is no matter which arbitrary record will the query use to return ungrouped values: they are all same within the group. A small test to show what differences in data can do for these queries. Group by clause use columns in Hive or relational database tables for grouping particular column values mentioned with the group by. Whenever you have a problem, please post a little sample data (CREATE TABLE and INSERT statements, relevant columns only) from all tables involved, so that the people who want to help you can re-create the problem and test their ideas. It's on a different site, but be sure to come back to sqlperformance.com right after.. One of the query comparisons that I showed in that post was between a GROUP BY and DISTINCT for a sub-query, showing that the DISTINCT is a lot slower, because it has to . Let me give you a short tutorial. The Moon turns into a black hole of the same mass -- what happens next? So unlike MyISAM, nothing bad happens in the first case here, and the GROUP BY query runs a little bit faster than the subqueries. The GROUP BY clause allows you to group rows based on values of one or more columns. Does there exist a Coriolis potential, just like there is a Centrifugal potential? BTW, adding my index may change the result. How do planetarium apps and software calculate positions? Subqueries are required to have names, which are added after parentheses the same way you would add an alias to a normal table. Can you give me some detail about how to interpret the execution plans in this specific case and which one is generally the preferable option? The other positive result of the second query is that only the uniqueness of ID_DestinationAddress is calculated, not the uniqueness of all the columns as a whole in the group by. I have a join that works but seems hacky because it orders the subquery and uses group by to filter everything but the top subquery result. A subquery is used to return data that will be used in the main query as a condition to further restrict the data to be retrieved. Returning a Scalar (single) Value A SPARQL Request String is a SPARQL Query String or SPARQL Update String and it is a Unicode character string (c.f. Moreover, a correlated subquery is executed repeatedly, once for each row . Why does "Software Updater" say when performing updates that it is "updating snaps" when in reality it is not? Step 1. Unfortunately, P.name LIKE '%tes%' requires scanning the entire table P. As for "ugly", I see both as ugly. There are a number of built-in expressions (documented below) that can be used to help you write queries. This query returns exactly same records as the previous one. Connect and share knowledge within a single location that is structured and easy to search. The first query is still untested. What do you think? Example #1 Find the number of employees in each department. Indent your code to make the clauses and sub-queries easy to recognize and understand. For example: Consider this query: This is really about just returning the publisher ids and one barcode (any) as an example from the products. Depending on the clause that contains it, a . 504), Hashgraph: The sustainable alternative to blockchain, Mobile app infrastructure being decommissioned. And then there are 10 supplier to show but the subquery still goes through all the rows and not just on the 10 suppliers found, Have you tried pushing the 'where' clause into the sub-query? I'll try to replicate your query plan when I have the time, @AndrReichelt added a small extra example. By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. Has Zodiacal light been observed from other locations than Earth&Moon? A correlated subquery is a subquery that uses the values of the outer query. As you can see, by using the subquery, you can combine two steps. On the one hand, the subqueries require some overhead and are executed several percents more slowly than the joins. SELECT AVG (salary) FROM employees; Code language: SQL (Structured Query Language) sql Second, the database system needs to evaluate the subquery only once. Logical Statements in SQL Three types 1) IIF () 2) CASE 3) CHOOSE MySQL's optimizer, however, does not take this into account. First, deactivate all products. How can I draw this figure in LaTeX with equations? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. The subquery removes the need for selecting the customer identification numbers and plugging them into the outer query. To execute any GROUP BY statement it should first order the records according to GROUP BY conditions. Every other engine would fail in this case, but MySQL allows selecting fields that are neighter grouped by nor aggregated. Can anyone help me identify this old computer part? By clicking Post Your Answer, you agree to our terms of service, privacy policy and cookie policy. However, most of my search results revolving around replacing subqueries involve JOINS. The correlated subquery may be preferable in the event it pr. If you'd like to learn more about useful SQL features you might not yet know, have a look at these slides: https://modern-sql.com/slides. Now, if I change my data so these 5 records have many more matches with dbo.Haul on the left join: The difference between the group by query. What is the difference between "INNER JOIN" and "OUTER JOIN"? What do you call a reply or comment that shows great quick wit? The performance of the OVER solution and the LATERAL/APPLY solution could vary (which is better depends on the data and indexes you have). It returns one row for each group. Feel free to ask questions and write me. How can I delete using INNER JOIN with SQL Server? The subqueries are notably faster (2 seconds for the subqueries against 4 seconds for the GROUP BY). However, due to some implementation issues, the subquery access requires some additional overhead which makes the subquery to run about 15% longer than its LEFT JOIN counterpart. Once the inner query runs, the outer query will run using the results from the inner query as its underlying table: SELECT sub.*. The GROUP BY statement groups rows that have the same values into summary rows, like "find the number of customers in each country". Here are a few examples to understand subqueries in the FROM clause. So the table scan and the PRIMARY KEY scan are in fact the same thing in InnoDB and there is no point in additional sorting. A SQL Server T-SQL correlated subquery is a special kind of temporary data store in which the result set for an inner query depends on the current row of its outer query. i want to achieve the same without the subquery. For InnoDB tables, the subqueries and the GROUP BY complete in almost same time, but GROUP BY is still several percent more efficient. Do this solution using WITH still hold weight today? Having said that, there is still one join that is not necessary anymore: the join of Products table to the result of the WITH clause (not the join on p.UnitPrice = mep.MaxUnitPrice. What do you call a reply or comment that shows great quick wit? Never write, let alone post, unformatted code. Replacing subqueries with JOIN or WITH. Also post the exact results you want from that data, and an explanation of how you get those results from that data, with specific examples. @AndrReichelt Nicely spotted, it got cut out for some reason, I re added it, thanks! Adding the where into the subquery would require a join there as well what is of course can be done and still better than nothing but I'd be curious if it could be solved without that Thanks, Fighting to balance identity and anonymity on the web(3) (Ep. Let's try the same queries, but now just return the first 100 records. Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. Products. If the intention is to show the most expensive product per category (only one in case of ties), the solution posted by @Tab Alleman while I was writing is OK. Having said that, I find the example pretty wired: it lists the most expensive products per category (which can be more then one in case the more products in a category having the same price). decode (fah.asset_type, 'NATU', fab.asset_cost_acct,fab.cip_cost_acct) asset_cost_acct, fl.segment1||'.'||fl.segment2||'.'||fl.segment3||'.'||fl.segment4||'. The table now looks like this: How can I draw this figure in LaTeX with equations? Enter your Username and Password and click on Log In Step 3. It might be that it does not matter in the end for your dataset, but I would prefer the second one nonetheless, just to get that stream aggregate earlier in the execution plan. "in the subquery" you define it outside the subquery. If you really need to list multiple rows in case of ties, you would use RANK() (or DENSE_RANK() instead of ROW_NUMBER() in @Tab Alleman's solution. Find centralized, trusted content and collaborate around the technologies you use most. This makes the optimizer to choose the table scan which is much faster when we need all records. FieldSort is the sort field that determines which records are in the top 5. This is because InnoDB tables are index-organized, and the PRIMARY KEY is the table itself. While a table join combines multiple tables into a new table, a subquery (enclosed in parentheses) selects rows from one table based on values in another table. But obviously yours is more succinct. thanks in advance. For MyISAM tables, the subqueries are often a better alternative to the GROUP BY. Select ( y => new { Result = 20 }) }). Concealing One's Identity from the Public When Purchasing a Home. '||fl.segment5) location, and BOOK_TYPE_CODE = 'IBM CORP')) BEGINNING_ASSET_BALANCE, DECODE (fah.asset_type, 'NATU', fab.asset_cost_acct,fab.cip_cost_acct). Neither quite conveys "just give me any barcode". Let's say we want to obtain the names and the costs of the products sold in our example. When the migration is complete, you will access your Teams at stackoverflowteams.com, and they will no longer appear in the left sidebar on stackoverflow.com. Answer (1 of 3): The correlated subquery may seem like a clumsy substitute for a JOIN query. The best answers are voted up and rise to the top, Not the answer you're looking for? How much better it is will depend on the unique value ratio of dbo.Haul. You don't want or need to put the alias inside the sub-query, doing so just means the outer-query ignores it: As I said above, you don't generally need to group by sub-queries, just the columns from the outer-query that the sub-query uses. Stack Exchange network consists of 182 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. But for InnoDB, these two access paths are in fact the same, since an InnoDB table is a PRIMARY KEY and the index traversal over a PRIMARY KEY is a table traversal. How is lift produced when the aircraft is going down steeply? To subscribe to this RSS feed, copy and paste this URL into your RSS reader. First, we need to select all users, even those who invited no other members. We remember that MySQL used the table scan with a sort for a query without a LIMIT, and an index scan for a query using a LIMIT. Unlike MyISAM, with InnoDB tables the optimizer chooses the index access path which avoids GROUP BY sorting. I would go even further than Paul in turning everything into joins. Asking for help, clarification, or responding to other answers. The tool and the set statistics command is very helpful. . To temper the ugliness, you say "barcode_sample". Which you appear to be doing, are you getting an error? One of the ways to test if you are not familiar with execution plans is setting SET STATISTICS IO, TIME ON; before executing the queries and making these runtime stats more readable by pasting them in a tool such as statisticsparser. For a non-square, is there a prime number for which it is a primitive root? If JWT tokens are stateless how does the auth server know a token is revoked? Don't miss. Formally, InnoDB is subject to the same optimizer mistake as the one described above for MyISAM. Taken together, they should make the index access path more efficient, and as we already saw in the previous section, they do, since normally LIMIT just takes first 10 records from the index. Updated queries and examine results with a WHERE clause. That's why in MyISAM the optimizer often makes incorrect decisions about whether or not use the index sort order or sort records taken from the table. Query Expressions. It was a comparison that showed that GROUP BY is generally a better option than DISTINCT. UPDATE product SET active = 'Y' WHERE price > ( SELECT AVG (price) FROM product ); This will set the active value to Y for all records that have a price above average. how do I use a value from the select in a joined subquery? Do conductor fill and continual usage wire ampacity derate stack? But be cautious of GROUP BY if you actually have more columns in the SELECT. This naturally returns records sorted by GROUP BY expressions and MySQL even cared to document this behavior. 600VDC measurement with Arduino (voltage divider), Stacking SMD capacitors on single footprint for power supply decoupling. Connect and share knowledge within a single location that is structured and easy to search. Thanks for contributing an answer to Database Administrators Stack Exchange! Stack Exchange network consists of 182 Q&A communities including Stack Overflow, the largest, most trusted online community for developers to learn, share their knowledge, and build their careers. Anyhow, I'm not sure why you need to join against a sub-select, I believe the following should work: Both need INDEX(publisher_id, barcode). Database Administrators Stack Exchange is a question and answer site for database professionals who wish to improve their database skills and learn from others in the community. In this case, optimizer makes the query to use the index even despite the fact that the table uses MyISAM, because LIMIT makes index traversal cheaper than sorting. My original, classic approach was to join both tables on a common ID, group by each field in the select list and order the result by the count of the sub table. Oracle evaluates this query in two steps: First, the subquery returns a list of the salesman whose sales is greater than or equal to 1 million. 1) DB2 first executes the subquery to get a list of publisher id: SELECT publisher_id FROM publishers WHERE name LIKE '%Oxford%'; Code language: SQL (Structured Query Language) (sql) Here is the output: PUBLISHER_ID ------------- 148 149 150 Code language: SQL (Structured Query Language) (sql) thanks a lot very helpful, even though im using postgres it gives me an idea where to start. In this case, the subquery returns to the outer query a list of values. Why? 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 following shows the syntax of the GROUP_CONCAT () function: GROUP_CONCAT ( DISTINCT expression ORDER BY expression SEPARATOR sep ); Code language: SQL (Structured Query Language) (sql) How did Space Shuttles get off the NASA Crawler? Which does almost the same but with a subquery join and group by. With InnoDB tables, optimizer makes no final filesort, so both solutions take almost the same time (albeit the subqueries are several percent less efficient again). For example: SELECT p1.site_name, (SELECT MAX (file_size) FROM pages p2 WHERE p1.site_id = p2.site_id) subquery2 FROM pages p1; With InnoDB, both queries complete in almost same time, but the GROUP BY query is still a little bit faster.

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