In this case, we cannot compare the performance between subquery and inner join since both queries have different output. the one with the ON clause). A join condition defines the way two tables are related in a query by: 1. It will be the same. Do what you’re comfortable with, as long as there isn’t a compelling reason (performance or otherwise) to do it a different way. I don't agree that this is an anti-pattern. So that’s what I write. UPDATE table_1 a INNER JOIN table_2 b ON b.id = SET a.value = b.value WHERE a.id = 3) join, both ids in ON clause. First, letâs assume there there is an index on department_id in both tables. We basically have the same logic as before: for highly selective filters, Oracle will use the employees table as the driving row source, otherwise it will pick (on) the departments table to take the lead. Using outer joins without need is rare mistake among people who start learning joins with comma syntax. 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. and How join query works internally? Whether the departments or employees table is used to generate an in-memory hash cluster depends on what table Oracle believes will be best based on the cardinality estimates available. The answer is: It depends! Ok, as Baron said, the two should be basically the same – in execution. If your boss is wildly deviating what he needs on a continual basis, he’s either incompetent, lazy, abusive, or just plain stupid. I thought having too much in the ON clause might slow down the query. Such an index is only beneficial to nested loops because that particular column is in the join clause. Such an index is only beneficial to nested loops because that particular column is in the join clause. Specifying a logical operator (for example, = or <>,) to be used in c… Tip: you can also use tuple comparison. If you need to refer the query which demonstrates the mentioned clauses, review following two queries for Join Better Performance. 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. That might be any of the available JOIN types, and any of the two access paths (table1 as Inner Table or as Outer Table). Please help or advise. @Salle, You can make both formats unreadable, no problem there. Simple db or complex db. Now, everyone recognizes it, and the few people who are still skeptical seem like dinosaurs. If no such indexes exist at all, then a hash join seems logical. The user was comparing 3 queries and wondered why the first took significantly longer than the other 2. If one prefers comma syntax, forcing an unfamiliar or non-preferred syntax on your code will in fact make it harder for that given developer to change it in the future. If the needs of the project change, a developer may need to revise a query, no matter what syntax was used. This means that on a complicated query with lots of table it is much more difficult to find the joining condition. why? When comma syntax is the first you are introduced to your brain establishes clear distinction between it and other types of joins. Filtering data They become too different and its difficult to forget that difference. Don’t forget the difference between “ON A.ID = B.ID” and “USING(ID)” – the first will give you the columns from both tables and the second will give you only the coalesced result of the two (as of 5.0.12, anyway). The splitting of these purposes with their respective clauses makes the query the most readable, it also prevents incorrect data being retrieved when using JOINs types other than INNER JOIN. 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. If the index on last_name is not selective at all and its clustering factor is closer to the number of rows than the number of blocks, then Query 2 may also be executed with a hash join, as we have discussed earlier. They’re going to be removed anyway. It’s opposite with people who learn Everything-About-SQL-Within-Two-Hours and their first touch with the term “join” is full list of all possible join types before even they come to the idea why joins are necessary at all. There are times when I need to change a inner join to left join and it is very difficult to explain the change to “those” programmers. Jerome, yes, you can use complex expressions in the ON clause. Consider this deliberately bad, but syntactically valid example: SELECT * FROM A JOIN B JOIN C ON (B.id = C.id) JOIN D ON (A.id = C.id AND D.id = B.id) JOIN E WHERE E.id = A.id AND A.x = ‘foo’ AND C.id = D.id AND D.y = ‘bar’; Is it more readable than equally bad style below? I have to agree on the readability. Posted on December 29, ... consisting of one table with one join and a simple WHERE clause. 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. The sort-merge join is rarely Oracleâs first choice when faced with equality join conditions, especially when the tables involved are not sorted to start with. 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. Percona's experts can maximize your application performance with our open source database support, managed services or consulting. MySQL, InnoDB, MariaDB and MongoDB are trademarks of their respective owners. Filtering results with the [ON] clause and the [Where] clause using LEFT OUTER JOIN and INNER JOIN is a very powerful technique. Also what is difference between nested loops and hash joins and how to determine which one to use from one of them for better performance ? Oracle SQL & PL/SQL Optimization for Developers. The goal is to create a query that … 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. +1! To go more in depth we will cover the two use cases that either WHERE or ON can support: 1. View query details This query returns all 10 values from the t_outerinstantly. Use Percona's Technical Forum to ask any follow-up questions on this blog topic. 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. Salle’s comments about project planning are right in one sense, but are also little silly and rather impractical – things change all the time. ; How the INNER JOIN works. Again, inner join returning more records than a subquery. I want to update a table with a value that has no apparent relationship with the table containing that value (no foreign key relationship), e.g. please explain with an example.Thanks,R Read “Join Processing Changes in MySQL 5.0.12” on http://dev.mysql.com/doc/refman/5.0/en/join.html . An INNER JOIN gives rows which match on the values in common columns of two or more tables using an operator like (=) equal.. A LEFT JOIN or LEFT OUTER JOIN gives all the rows from the left table with matched rows from both tables. 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? However it seems the only way (1) would perform as well as (2) is if the query optimizer reduced them to the same internal operations. By using joins, you can retrieve data from two or more tables based on logical relationships between the tables. A join clause is used to combine records or to manipulate the records from two or more tables through a join condition. ON should be used to define the join condition and WHERE should be used to filter the data. 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. A typical join condition specifies a foreign key from one table and its associated key in the other table. can i add many conditions or rather leaving it to max one? So, to optimize performance, you need to be smart in using and selecting which one of the operators. INNER JOINs and the effect on performance. I have a query design question related to using CASE statements vs. On the other hand, when you use JOINS you might not get the same result set as in the IN and the EXISTS clauses. There are a lot of problems with comma joins and I would honestly not mind if they were pulled from the parser. Yes it’s true that comma syntax makes it easier to write unreadable code, but the issue about readability is bit exaggerated. 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. I prefer it myself. Oracleis smart enough to make three logical constructs: 1. 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. However if you want to make the most readable code possible, it’s hard to argue that comma syntax can be made to be more readable than ON syntax. But I guess in those cases you would just call your boss an ass for not figuring out what he needed at the planning stages of the project???? (+) seemed really natural . As such, ... 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. Here are the queries: Query 1: SELECT * From TabA INNER JOIN TabB on TabA.Id=TabB.Id Query 2: SELECT * From TabA WHERE Id in (SELECT Id FROM TabB) Query 3: SELECT TabA. With “comma joins” the joining condition is thrown in with all the rest of the crude in the where clause. 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. If the tables are not big enough, or there are other reasons why the optimizer doesn't expand the queries, then you might see small differences. If I change the sequence of joins , it doesn’t work but if i change the sequence in where clause of comma separated query, It works. The behavior has changed significantly between 4.X and when 5.0. To mix up more on the confusion, it’s basically not about readability or syntax construction but rather the goal of the process. You can optimizing joins by choosing proper indexes, just like searching in the WHERE clause. If you don’t care about readability the language per se doesn’t help. When the predicate is in the ON clause, you filter the rows from T2, then outer join those results to T1. To join table A with the table B, you follow these steps:. But every so often, I’m surprised by someone who says they actually prefer the Oracle proprietary outer join syntax. EXISTS vs IN vs JOIN with NOT NULLable columns: Logically I'd You can write unclear code no matter whether it’s on single or multiple lines. Therefore the SQL reserved word JOIN doesn’t map to any natural language word as it would be the case if I was using “join” in everyday language. If we look into the query plan we will see that this is just a plain NESTED LOOPSjoin on the index. 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. ; Second, specify the main table i.e., table A in the FROM clause. So, if you need to adjust the query such that limitations on either sides of the tables should be in-place, the JOIN is more preferred: SELECT * FROM A LEFT OUTER JOIN B ON A.id=B.id WHERE A.x=123; So in turn, the comma syntax will have to re-code the whole structure and adopt the join syntax instead. JOIN performance has a lot to do with how many rows you can stuff in a data page. Virtually any expression that would work in a WHERE clause is okay for an ON clause. Being one of the dinosaurs who prefer comma syntax I’d like to say few words on that. I hope this blog illustrating SQL join vs union, SQL join vs subquery etc was helpful to you. Joins indicate how SQL Server should use data from one table to select the rows in another table. 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. Two of the calculated columns used a SQL IN clause on the @users table; something to this effect: WHERE id IN ( SELECT u.id FROM @users u ) Since these were one-to-one type relationships (only one record per-user-id in the parent query), I changed the IN clauses to INNER JOIN clauses. I’d think the only place you’d see a difference would be where the isolated logic in the ANSI syntax saved mysql from doing something you didn’t intend for it to do, like join in the wrong place. Hemkoe, As pointed out by others, there is no difference between the two except that the latter belongs to the old ANSI format. Valid for human languages too not only programming ones. In other words, you could expect equal performance. Queries 2 and 3 yield different result sets, so itâs more or less comparing apples and oranges. You’d better publish some time score on a simple DB test ?! Third, specify the second table (table B) in the INNER JOIN clause and provide a join condition after the ON keyword. 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. In addition to these points, the type of join used in a query implies intent. It used to be a huge uphill battle to get people even to understand the syntax, and they remained unwilling to use it. of customer actvity, and then your boss says, “okay now show all customers, including those who have no activity.” Or another example: “include all customers you had before, but restrict the totals to their activity during a certain time span.” Those are both realistic examples of when you’d change an inner join to an outer join for a given query. The point is partially that the boss will, if they’re competent, know the business requirements during the planning phase — and if you’re competent you’ll be able to articulate those requirements into code. We can say that their logical working is different. If you start your career using Oracle and then work mostly on your own person projects… Now that I’m on MySql I still have to look up that join syntax up on wikipedia! Both examples are deliberately bad ones, but in practice we often see queries much worst than that. I have seen a lot of easy to read and understand perl scripts for example and lot of completely unreadable sources in languages supposedly much easier to read. If the code accomplishes it’s aims and is able to be maintained, then you’re doing it “right”. Salle is right – join in where rules! SELECT orders.id, buyers.name AS Buyers_name, products.name AS Product_name, orders.price AS Purchased_Price, orders.created_at AS Purchased_Date, sellers.name AS Seller_name, sellerproducts.price AS Current Price, LEFT JOIN buyers ON (buyers.id = orders.buyer_id) LEFT JOIN sellerproducts ON (sellerproducts.id = orders.sellerproduct_id) LEFT JOIN products ON (products.id = sellerproducts.product_id) LEFT JOIN sellers ON (sellers.id = sellerproducts.seller_id), FROM orders , buyers, products, sellers , sellerproducts, WHERE buyers.id = orders.buyer_id and products.id = sellerproducts.product_id and sellerproducts.id = orders.sellerproduct_id and sellers.id = sellerproducts.seller_id. Yes. SQL inner join vs subquery. Period!” (I argued once with someone who was claiming that and his “proof” was also “they told me”). The core logic of the query can change If you apply the filter in WHERE clause OR JOIN clause, the join can no longer remain an outer join and could become an inner join in case of LEFT/RIGHT outer join OR become left/right outer join in case of FULL OUTER JOIN Let me give you some TSQL examples. If you have a where on the table you're outer joining to, you effectively make the query an inner join. The comma syntax comes easy on such: SELECT * FROM A,B WHERE A.id = B.id and A.x=123; But, the join syntax will help it to be more flexible when necessary. The problem was that this query was taking over 11 minutes to run, and only returned about 40,000 results. 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. Best to test both. Shouldn't the query planner be smart enough to know that the first query is the same as the second and third? Thanks a bunch Jerome. You can save some time by running EXPLAIN EXTENDED, then SHOW WARNINGS, and examine the reconstructed query. I would also note the “coma join” is equivalent to JOIN not “LEFT JOIN” I see a lot of people use LEFT JOIN with no reason and this does restrict execution plan choices. The database will do so either with a lookup if the relevant index on employees is selective enough or by means of a full table scan if it is not highly selective. 1) no join, and both ids in where clause. Also subquery returning duplicate recodes. Salle: The issue about readability is not exaggerated. 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. It’s common to write an SQL query for a report e.g. No, there’s no difference. Most of the time, IN and EXISTS give you the same results with the same performance. Why is that this the case? 1. The execution plan is the same, but the extra bytes going over the wire might add up to some difference if the result set is large enough. They are all the same aren’t they?). Exactly my point Bill. 2) join, id of table to be updated in ON clause, the other id in where clause. Hi there, I am using a LEFT JOIN and i was wondering if having many condition – ie. 1. Nevertheless, with an appropriate, selective index on last_name Oracle will probably settle for nested loops for Query 2 (i.e. He is a former Percona employee. 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. Baron is the lead author of High Performance MySQL. This is not a recommended habit in any language. * Even if you think the issue about readability is a bit exaggerated, (which is not because really long and complex queries that you might not come across), it is an issue, so it makes absolutely no sense of using comma syntax just for that simple fact. Bill’s example is the kind of thing that happens ALL the time. Hi Bill, Thanks a lot for helping me. 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. 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. Rob: You are missing the fact that JOIN .. ON .. syntax does *not* prevent having join conditions buried somewhere in WHERE clause in case of inner joins. Even if you know what you’re looking for it is messy and difficult to understand at a glance. I spend a lot of my time looking at other peoples queries and I have regex to add white space for readability. 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. In that case just for fun guess one option LEFT JOIN or NOT IN. Thanks again for your help. Welcome to the real world? Same example with sample schemas OE But that’s just personal preference. Here are perfectly valid syntax examples: SELECT * FROM A JOIN B INNER JOIN C INNER JOIN D JOIN E; SELECT * FROM A JOIN B JOIN C JOIN D JOIN E ON (A.id = C.id) WHERE D.id = B.id; 2 years later but I would like to point out some misconceptions you have. “Is there a performance difference between putting the JOIN conditions in the ON clause or the WHERE clause in MySQL?”. SELECT * FROM table1 LEFT JOIN table2 ON table2.x=table1.y AND table2.b=table1.a. It is not just about readability. It will then use the index on departments to access its data by ROWID, thereby joining it to the data from the leading row source. @Salle: I disagree. My point is that if you don’t care about readability this syntax does not help. In this puzzle, we’re going to learn how to rewrite a subquery using inner joins. * Finally, you write 2 perfectly valid syntax examples (Which are not), and none of those are comma syntax. (Inner, outer, natural .. who cares? IN is equivalent to a JOIN / DISTINCT 2. On Tue, 19 Dec 2006 20:02:35 +0000, "Richard Huxton"
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