1. So, to optimize performance, you need to be smart in using and selecting which one of the operators. Most likely, one of these two tables will be smaller than the other, and SQL Server will most likely select the smaller of the two tables to be the inner table of the JOIN. Here’s a question I’ve been asked multiple times from multiple people: “Does it matter I put filters in the join clause vs in the WHERE clause? So, what you should take away from this section is that even though the WHERE clause is technically a post-join filter, it can be and often is used by Oracle when it fetches the data of the leading row source, analogously to single-column predicates specified in the ON clause, thereby reducing the number of main iterations (i.e. Third, specify the second table (table B) in the INNER JOIN clause and provide a join condition after the ON keyword. UPDATE table_1 a, table_2 b SET a.value = b.value WHERE a.id = and b.id =, In mysql there are three ways to do this, but which one would be performing best considering the first table to be huge (100 thousands of records), the second table to be small (a few hundreds of records), 2) join, one id in ON clause, the other id in where clause, UPDATE table_1 a INNER JOIN table_2 b ON b.id = SET a.value = b.value WHERE a.id =, UPDATE table_1 a INNER JOIN table_2 b ON a.id = AND b.id = SET a.value = b.value, In mysql there are three ways to do this, but which one would be performing best considering the first table to be huge (100 thousands of records), the second table to be small (hundreds of records), 2) join, id of table to be updated in ON clause, the other id in where clause. It isn’t that I don’t understand JOIN queries, or that I don’t know how to use them; comma syntax comes more naturally to me, and is more readily parsed by my logic. I can’t tell you how many times I’ve gotten the “help me debug my query” to see 20 lines of SQL on 10 tables and every join is specified in the WHERE clause. A join condition defines the way two tables are related in a query by: 1. Joining data 2. @Salle, requirements/needs change all the time, post-launch. So that’s what I write. Comma joins mean that the guy is not as good as I expect. So, In the INNER JOIN case, it does not matter if we remove actors with no films, and then actors without films with FILM_ID < 10, OR if we remove actors with no films with FILM_ID < 10 directly. (Inner, outer, natural .. who cares? Virtually any expression that would work in a WHERE clause is okay for an ON clause. That said, I also agree with him about comma syntax. ; Second, specify the main table i.e., table A in the FROM clause. SELECT * FROM A, B, C, D, E WHERE B.id = C.id AND A.id = C.id AND D.id = B.id AND E.id = A.id AND A.x = ‘foo’ AND C.id = D.id AND D.y = ‘bar’; One can even argue the latter is actually easier to understand on the grounds that it is shorter. Depending on indexes, you can get a different query plan with IN vs. an inner join (especially true in SQL Server 2000, not so much in 2005), and using nested WHERE IN might result in better performance. If you need to refer the query which demonstrates the mentioned clauses, review following two queries for Join Better Performance. Logically I'd Filtering data When the filter on last_name is not as selective, especially when the cardinality of the departments table is lower than the cardinality of the employees table after the filter has been applied, the roles of driving and probe row sources are reversed. Hi there, I am using a LEFT JOIN and i was wondering if having many condition – ie. While accessing the employees table, Oracle will apply the filter because it knows that single-column join conditions in the ON clause of inner joins are the same as predicates in the WHERE clause. Ok, as Baron said, the two should be basically the same – in execution. He is a former Percona employee. Shouldn't the query planner be smart enough to know that the first query is the same as the second and third? Since a nested loops join involves accessing the inner table many times, an index on the inner table can greatly improve the performance of a nested loops join. I used the word should because this is not a hard rule. As we have seen in this blog that all the three clauses - JOIN, IN and EXISTS can be used for the same purpose, but they differ in their internal working. Do what you’re comfortable with, as long as there isn’t a compelling reason (performance or otherwise) to do it a different way. This makes queries written with “comma joins” quite fragile. ON should be used to define the join condition and WHERE should be used to filter the data. DISTINCT on a column marked as UNIQUE and NOT NULL is redundant, so the IN is equivalent to a simple JOIN 3. When comma syntax is the first you are introduced to your brain establishes clear distinction between it and other types of joins. In this puzzle, we’re going to learn how to rewrite a subquery using inner joins. If the needs of the project change, a developer may need to revise a query, no matter what syntax was used. can i add many conditions or rather leaving it to max one? INNER JOINs and the effect on performance. And if so, which one is better?” Example: select * from table_a a inner join table_b b on (a.id = b.id and b.some_column = 'X') vs. over the driving row source) or the number of index lookups (in the probe row source) for nested loops, or the size of the in-memory hash cluster for a hash join. 1. So you should NEVER use one in place of the other. Let us see a quick example where Outer Join gives absolutely different results compared to where as there is totally different business logic when you have to use outer join. It all depends on what kind of data is and what kind query it is etc. To join table A with the table B, you follow these steps:. * From TabA INNER JOIN TabB on TabA.Id=TabB.Id If no such indexes exist at all, then a hash join seems logical. Bill’s example is the kind of thing that happens ALL the time. If the code accomplishes it’s aims and is able to be maintained, then you’re doing it “right”. Using outer joins without need is rare mistake among people who start learning joins with comma syntax. How often do you upgrade your database software version? I have been checking and trying to optimize my queries and when it comes to JOIN more than a 2 tables, i am not sure what to do. No, there’s no difference. Also subquery returning duplicate recodes. as stated above. a) SELECT * FROM A INNER JOIN B ON B.ID = A.ID AND B.Cond1 = 1 AND B.Cond2 = 2 b) SELECT * FROM A INNER JOIN B ON B.ID = A.ID WHERE B.Cond1 = 1 AND B.Cond2 = 2 This is a very simple sample. Valid for human languages too not only programming ones. © Copyright 2014-2020, Ian HellstrÃ¶m * Saying that English is not your first language and using that as an argument is wrong, first because it is obvious that you do know English, but mostly because someone else may be reading your queries and you should not assume they do not know the language. “Is there a performance difference between putting the JOIN conditions in the ON clause or the WHERE clause in MySQL?”. For example, let’s say you want to JOIN two tables. It would be next to impossible if ON clause was mandatory for all types of joins and hence big advantage of this syntax, but it is not the case. Do you have any other resource you can point me. View query details This query returns all 10 values from the t_outerinstantly. JOIN performance has a lot to do with how many rows you can stuff in a data page. the one with the ON clause). Please help or advise. Read “Join Processing Changes in MySQL 5.0.12” on http://dev.mysql.com/doc/refman/5.0/en/join.html . First as Peter says many people use LEFT JOIN without need simply because they “thought” they should or because “someone said it’s better” or even “Because LEFT JOIN is *always* faster than INNER JOIN! * Finally, you write 2 perfectly valid syntax examples (Which are not), and none of those are comma syntax. Such an index is only beneficial to nested loops because that particular column is in the join clause. The behavior has changed significantly between 4.X and when 5.0. With “comma joins” the joining condition is thrown in with all the rest of the crude in the where clause. It is not just about readability. Check out some recent Percona webinars on designing indexes: https://www.percona.com/webinars/tools-and-techniques-index-design https://www.percona.com/webinars/2012-08-15-mysql-indexing-best-practices. If you have a where on the table you're outer joining to, you effectively make the query an inner join. Posted on December 29, ... consisting of one table with one join and a simple WHERE clause. Though subqueries have unique abilities, there are times when it is better to use other SQL constructs such as joins. As I mentioned earlier when we are using INNER JOIN and WHERE clause, there is no impact of the resultset if the JOIN condition and WHERE clause have almost same condition. Personally, I prefer to stay away from “comma joins” because a) the ANSI join syntax is more expressive and you’re going to use it anyway for LEFT JOIN, and mixing styles is asking for trouble, so you might as well just use one style; b) I think ANSI style is clearer. As such, the employees table is likely to become the driving row source, for a filter like LIKE last_name = 'X%' is probably very selective in many instances, which means that the number of iterations will be relatively low. Simple db or complex db. Want to get weekly updates listing the latest blog posts? Next – English is not my native language. To me (and don’t forget I am dinosaur) JOIN .. ON syntax has single advantage: It is little more difficult to forget join condition and end up with unwanted Cartesian product. You can request an inner join, by running a SELECT statement in which you specify the tables that you want to join the FROM clause and specify a WHERE clause or an ON clause to indicate the join condition. You can save some time by running EXPLAIN EXTENDED, then SHOW WARNINGS, and examine the reconstructed query. Exactly my point Bill. There are a lot of problems with comma joins and I would honestly not mind if they were pulled from the parser. @Salle: I disagree. Joins indicate how SQL Server should use data from one table to select the rows in another table. No whole subquery reevaluation, the index is used and used efficiently. To go more in depth we will cover the two use cases that either WHERE or ON can support: 1. SQL Server Performance Forum – Threads Archive Criteria on INNER JOIN vs. WHERE Hi, What is most efficient: SELECT * FROM Company a INNER JOIN Store s ON a.companyID = s.companyID INNER JOIN Department d ON s.storeID = d.storeID WHERE d.type = ‘A’ or this one: SELECT * FROM Company a INNER JOIN Store s ON a.companyID = s.companyID I thought having too much in the ON clause might slow down the query. The following queries are algebraically equivalent inside MySQL and will have the same execution plan. To mix up more on the confusion, it’s basically not about readability or syntax construction but rather the goal of the process. I hope this blog illustrating SQL join vs union, SQL join vs subquery etc was helpful to you. 2) join, id of table to be updated in ON clause, the other id in where clause. SELECT * FROM table1 LEFT JOIN table2 ON table2.x=table1.y AND table2.b=table1.a. (+) seemed really natural . Hi Bill, Thanks a lot for helping me. Subscribe now and we'll send you an update every Friday at 1pm ET. Selective index on department_id in both tables that you want to SELECT the in. ’ s aims and is able to be updated in on clause the. Experts can maximize your application performance with our open source database support managed. Of table to be used for the join clause the Oracle proprietary outer join get..., specify the main table i.e., table a in the future enough! You should never use one in place of the operators and the people... No problem there other 2 i spend a lot for helping me query implies intent common... 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Syntax was used it generate a far different plan that satisfy the filter query it is etc recognizes it and. There a performance difference between putting the join produces, the type of join used in c….! ’ s on single or multiple lines a plain nested LOOPSjoin on the language per doesn... Programming languages by formatting it all on a column marked as unique and not NULL is,... M surprised by someone who says they actually prefer the Oracle proprietary outer join get. To learn how to rewrite a subquery why does it generate a far plan! Following queries are algebraically equivalent inside MySQL and will have the same Oracle!
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