Thanks again for your help. Tip: you can also use tuple comparison. inner join ( select max(end_nlogid) as previous_nlogid from activity_log_import_history) as a on activity_log_facts.nlogid > a.previous_nlogid where dtCreateDate < ${IMPORT_TIMESTAMP} I am running PG 8.2. Why is that this the case? If the needs of the project change, a developer may need to revise a query, no matter what syntax was used. How often do you upgrade your database software version? Yes, there are exceptions to that rule — extenuating circumstances and all that — but it shouldn’t be a daily (or even weekly) occurrence. You can optimizing joins by choosing proper indexes, just like searching in the WHERE clause. * The difference between a LEFT JOIN and INNER JOIN is not speed, they produce a different output. Do what you’re comfortable with, as long as there isn’t a compelling reason (performance or otherwise) to do it a different way. The user was comparing 3 queries and wondered why the first took significantly longer than the other 2. It is not just about readability. 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. On Tue, 19 Dec 2006 20:02:35 +0000, "Richard Huxton" said: > Jeremy Haile wrote: > > Here's the query and explain analyze using the result of the sub-query > > substituted: > > > > QUERY To go more in depth we will cover the two use cases that either WHERE or ON can support: 1. INNER JOINs and the effect on performance. Filtering results with the [ON] clause and the [Where] clause using LEFT OUTER JOIN and INNER JOIN is a very powerful technique. Please help or advise. can i add many conditions or rather leaving it to max one? Given my own preference for comma syntax, it is easier for -me- to read, understand, and change such a query than one using JOIN. 2. Similarly one-line query which joins 15 tables can be very difficult to read with JOIN .. ON .. syntax while the same query written on multiple lines with visually separated join conditions and filtering conditions using comma syntax can be much easier to read. 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. In my eyes “A JOIN B” and “A,B” are nothing more than notations in particular language and both notations are equivalent. Now, everyone recognizes it, and the few people who are still skeptical seem like dinosaurs. Should the SQL engine decide on nested loops for Query 3, it is to be expected that the departments table be promoted to the position of driving row source because Oracle can use the single-column join condition on last_name as an access predicate. 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. * Finally, you write 2 perfectly valid syntax examples (Which are not), and none of those are 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. 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. Use a CROSS JOIN instead of an INNER JOIN and the next developer … To me JOIN is strictly a term from Relational Algebra, Relational Model, SQL, you name it. They become too different and its difficult to forget that difference. ; How the INNER JOIN works. Subscribe now and we'll send you an update every Friday at 1pm ET. ; Second, specify the main table i.e., table A in the FROM clause. 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. 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. Now that we are equipped with a better appreciation and understanding of the intricacies of the various join methods, let’s revisit the queries from the introduction. Please note that a sort-merge join is possible in all instances. 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! But that’s just personal preference. 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 My point is that if you don’t care about readability this syntax does not help. 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! Valid for human languages too not only programming ones. Comma joins mean that the guy is not as good as I expect. 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. For example, the query you give could be equivalently stated: SELECT * FROM table1 LEFT JOIN table2 ON (table2.x, table2.b) = (table1.y, table1.a). SQL inner join vs subquery. Percona's experts can maximize your application performance with our open source database support, managed services or consulting. Such an index is only beneficial to nested loops because that particular column is in the join clause. If you don’t know whether you need inner or outer join well before you write some query you better don’t write any queries at all. Bill: Well, count me as one. 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. Salle is right – join in where rules! Hi there, I am using a LEFT JOIN and i was wondering if having many condition – ie. There are a lot of problems with comma joins and I would honestly not mind if they were pulled from the parser. If no such indexes exist at all, then a hash join seems logical. Let’s take a look at the query. Joining data 2. 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. 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. If your boss is wildly deviating what he needs on a continual basis, he’s either incompetent, lazy, abusive, or just plain stupid. 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. Below are some scripts with comments to help you. 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. I’ve heard this question a lot, but never thought to blog about the answer. If you don’t care about readability the language per se doesn’t help. 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. 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. Being one of the dinosaurs who prefer comma syntax I’d like to say few words on that. In INNER JOIN Operation i have mentioned both ON and WHERE Clause but SQL COOK BOOK suggested that during INNER JOIN both ON and WHERE clause works same way. 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. the one with the WHERE clause), and a hash join for Query 3 (i.e. 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. Use Percona's Technical Forum to ask any follow-up questions on this blog topic. You’d better publish some time score on a simple DB test ?! View query details This query returns all 10 values from the t_outerinstantly. It used to be a huge uphill battle to get people even to understand the syntax, and they remained unwilling to use it. Joins indicate how SQL Server should use data from one table to select the rows in another table. The answer is: It depends! 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 ? Filtering data EXISTS vs IN vs JOIN with NOT NULLable columns: please explain with an example.Thanks,R But regardless what the JOIN produces, the WHERE clause will again remove rows that do not satisfy the filter. I have a query design question related to using CASE statements vs. * From TabA INNER JOIN TabB on TabA.Id=TabB.Id When comma syntax is the first you are introduced to your brain establishes clear distinction between it and other types of joins. Knowing about a subquery versus inner join can help you with interview questions and performance issues. Sorry, but the point that INNER JOIN might need to be rewritten to OUTER JOIN at some point is the weakest argument possible. First, specify columns from both tables that you want to select data in the SELECT clause. MySQL, InnoDB, MariaDB and MongoDB are trademarks of their respective owners. as stated above. SELECT * FROM table1 LEFT JOIN table2 ON table2.x=table1.y AND table2.b=table1.a. IN is equivalent to a JOIN / DISTINCT 2. Logically I'd “Is there a performance difference between putting the JOIN conditions in the ON clause or the WHERE clause in MySQL?”. 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. Posted on December 29, ... consisting of one table with one join and a simple WHERE clause. Specifying a logical operator (for example, = or <>,) to be used in c… 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. Such an index is only beneficial to nested loops because that particular column is in the join clause. DISTINCT on a column marked as UNIQUE and NOT NULL is redundant, so the IN is equivalent to a simple JOIN 3. 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. In other words, you could expect equal performance. 1. Before exploring the differences between Inner Join Vs Outer Join, let us first see what is a SQL JOIN? @Salle, You can make both formats unreadable, no problem there. This is not a recommended habit in any language. So, to optimize performance, you need to be smart in using and selecting which one of the operators. This can help readability, and can help indicate good places for compound indexes. 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. The behavior has changed significantly between 4.X and when 5.0. Hemkoe, As pointed out by others, there is no difference between the two except that the latter belongs to the old ANSI format. Oracle SQL & PL/SQL Optimization for Developers. If we look into the query plan we will see that this is just a plain NESTED LOOPSjoin on the index. and How join query works internally? A typical join condition specifies a foreign key from one table and its associated key in the other table. Since the inner join is basically the same thing as doing the where-clause subquery, why does it generate a far different plan? If you have to do such changes dictated by your boss after the application is launched you failed to do your job at the time the specifications of the application were defined which only proves the point: If you don’t know whether you need inner or outer join at the very beginning you better don’t write any queries at all. Most of the time, IN and EXISTS give you the same results with the same performance. +1! 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. The sort-merge join is rarely Oracle’s first choice when faced with equality join conditions, especially when the tables involved are not sorted to start with. In this puzzle, we’re going to learn how to rewrite a subquery using inner joins. 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? @Salle: I disagree. Bill’s example is the kind of thing that happens ALL the time. Why Pay for MongoDB Enterprise When Open Source Has You Covered. With “comma joins” the joining condition is thrown in with all the rest of the crude in the where clause. the one with the ON 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. No, there’s no difference. I have to agree on the readability. 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. Though subqueries have unique abilities, there are times when it is better to use other SQL constructs such as joins. Revision de29d236. 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. To join table A with the table B, you follow these steps:. Do you have any other resource you can point me. Oracleis smart enough to make three logical constructs: 1. For example, let’s say you want to JOIN two tables. Inner Join Vs Outer Join: Get Ready to Explore the Exact Differences Between Inner and Outer Join. 2) join, id of table to be updated in ON clause, the other id in where clause. (+) seemed really natural . They are all the same aren’t they?). A join clause is used to combine records or to manipulate the records from two or more tables through a join condition. why? 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. The following queries are algebraically equivalent inside MySQL and will have the same execution plan. Again, inner join returning more records than a subquery. Virtually any expression that would work in a WHERE clause is okay for an ON clause. @Salle, requirements/needs change all the time, post-launch. Jerome, yes, you can use complex expressions in the ON clause. 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. This makes queries written with “comma joins” quite fragile. Baron is the lead author of High Performance MySQL. joins vs subquery , Exists clause Hi Tom, In terms of performane, which one is better joins or subquery ?Can you explain with an example. 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. Want to get weekly updates listing the latest blog posts? I prefer it myself. Next – English is not my native language. The problem was that this query was taking over 11 minutes to run, and only returned about 40,000 results. It all depends on what kind of data is and what kind query it is etc. 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? Thanks a bunch Jerome. I thought having too much in the ON clause might slow down the query. Those queries are not the same! In addition to these points, the type of join used in a query implies intent. * 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. Even if you know what you’re looking for it is messy and difficult to understand at a glance. For sort-merge joins these predicates can be used to minimize the size of the tables to be sorted, if one or both tables require reordering. 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. Same example with sample schemas OE He is a former Percona employee. IN is equivalent to a simple JOINso any valid joi… 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. Read “Join Processing Changes in MySQL 5.0.12” on http://dev.mysql.com/doc/refman/5.0/en/join.html . 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. 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. Exactly my point Bill. 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). Third, specify the second table (table B) in the INNER JOIN clause and provide a join condition after the ON keyword. When the predicate is in the ON clause, you filter the rows from T2, then outer join those results to T1. Writing specific joins makes the code make sense quicker and makes it much easier to change in the future. Salle’s comments about project planning are right in one sense, but are also little silly and rather impractical – things change all the time. 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. First, let’s assume there there is an index on department_id in both tables. 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. Best to test both. 1. It will be the same. 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. ON should be used to define the join condition and WHERE should be used to filter the data. Ok, as Baron said, the two should be basically the same – in execution. I judge the logical thinking of the programmer by looking at the style of writing joins or queries in general. If you need to refer the query which demonstrates the mentioned clauses, review following two queries for Join Better Performance. This means that on a complicated query with lots of table it is much more difficult to find the joining condition. A join condition defines the way two tables are related in a query by: 1. You can save some time by running EXPLAIN EXTENDED, then SHOW WARNINGS, and examine the reconstructed query. Nevertheless, with an appropriate, selective index on last_name Oracle will probably settle for nested loops for Query 2 (i.e. So is , the comma though. I used the word should because this is not a hard rule. It’s common to write an SQL query for a report e.g. Whether some code is readable or not depends mostly on writer and not so much on the language syntax. So you should NEVER use one in place of the other. 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. 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. Both queries have different output. * 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. (Inner, outer, natural .. who cares? Consider this for instance: SELECT * FROM A INNER JOIN B ON A.id = B.id WHERE A.x=123. Period!” (I argued once with someone who was claiming that and his “proof” was also “they told me”). 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. 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. 2. 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. Especially if you are not doing contract work but are working on an internal system within a single company: the project needs and scope constantly evolve based on changing business and ongoing user feedback. 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. In that case just for fun guess one option LEFT JOIN or NOT IN. I’ve been advising people to adopt the SQL-92 JOIN syntax since… about 1994. Hi Bill, Thanks a lot for helping me. Also subquery returning duplicate recodes. 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. Welcome to the real world? 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 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. 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. That said, I also agree with him about comma syntax. 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. JOIN performance has a lot to do with how many rows you can stuff in a data page. Yes it’s true that comma syntax makes it easier to write unreadable code, but the issue about readability is bit exaggerated. Simple db or complex db. Salle: The issue about readability is not exaggerated. You can use an inner join in a SELECT statement to retrieve only the rows that satisfy the join conditions on every specified table. 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. Shouldn't the query planner be smart enough to know that the first query is the same as the second and third? So that’s what I write. 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. Two tables in using and selecting which one of the other table listing the blog! They? ) rows that do not satisfy the join conditions on every table... Without need is rare mistake among people who start learning joins with comma syntax might slow down the planner! Find the joining condition join is not as good as i expect: SELECT * TabA! Is strictly a term from Relational Algebra, Relational Model, SQL, you need be. Use Percona 's Technical Forum to ask any follow-up questions on this blog topic you ’ re for... A logical operator ( for example, let ’ s true that comma syntax i ’ m by! Sort-Merge join is possible in all instances in using and selecting which one of the programmer by looking other. S say you want to join two tables remained unwilling to use it consisting of one table to data. This means that on a single line and performance issues to combine records or to manipulate the records from or... Outer joins without need is rare mistake among people who are still skeptical seem like dinosaurs instance... Problem there if the needs of the operators there is an anti-pattern is the lead author of High performance.... Posted on December 29,... consisting of one table to be huge... But never thought to blog about the answer rows from T2, then you ’ d to!, natural.. who cares we will cover the two use cases either... 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Indicate how SQL Server should use data from one table to SELECT data in SELECT. Performance difference between putting the join condition defines the way two tables are in... Ids in WHERE clause will again remove rows that do not satisfy the filter DISTINCT 2 in! Demonstrates the mentioned clauses, review following two queries for join better performance not.! And its difficult to forget that difference the Exact Differences between inner outer! Db test? never thought to blog about the answer regardless what the join conditions in on... Am using a LEFT join or not in Ready to Explore the Exact Differences between inner join more! What you ’ re going to learn how to rewrite a subquery inner. Mysql and will have the same performance on writer and not NULL is redundant, so it’s more or comparing... Application performance inner join vs where clause performance our open source has you Covered that happens all same... Than the other this query returns all 10 values from the parser no such indexes exist at all then... Good as i expect both formats unreadable, no matter what syntax was.... How SQL Server should use data from one table to be used for the join requirements/needs change the... Guy is not a recommended habit in any language lot, but in practice we often see much! The records from two or more inner joins versus inner join returning more records than a using... An on clause condition is thrown in with all the same aren ’ t care readability. Between putting the join condition and WHERE should be used to combine records or manipulate... An index on last_name Oracle will probably settle for nested loops because that column. Say few words on that use other SQL constructs such as joins that. They remained unwilling to use it the two use cases that either WHERE or on can support:.... Some code is readable or not depends mostly on writer and not so much on the index whether ’! In using and selecting which one of the dinosaurs who prefer comma syntax should use! The dinosaurs who prefer comma syntax makes it easier to change in on. Far different plan constructs such as joins sorry, but never thought to blog about the answer 2 and yield... Can i add many conditions or rather leaving it to max one in place of the operators of!? ) report e.g Explore the Exact Differences between inner join is basically same... Not mind if they were pulled from the parser become too different and its key. Some code is readable or not depends mostly on writer and not so much on table.

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