ON should be used to define the join condition and WHERE should be used to filter the data. 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. 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. 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. 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. 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! To mix up more on the confusion, it’s basically not about readability or syntax construction but rather the goal of the process. why? 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. 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. If you don’t care about readability the language per se doesn’t help. Before exploring the differences between Inner Join Vs Outer Join, let us first see what is a SQL JOIN? Jerome, yes, you can use complex expressions in the ON clause. 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. Such an index is only beneficial to nested loops because that particular column is in the join clause. Using outer joins without need is rare mistake among people who start learning joins with comma syntax. 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. please explain with an example.Thanks,R Filtering results with the [ON] clause and the [Where] clause using LEFT OUTER JOIN and INNER JOIN is a very powerful technique. By using joins, you can retrieve data from two or more tables based on logical relationships between the tables. So that’s what I write. Posted on December 29, ... consisting of one table with one join and a simple WHERE clause. If no such indexes exist at all, then a hash join seems logical. Salle is right – join in where rules! On the other hand, when you use JOINS you might not get the same result set as in the IN and the EXISTS clauses. Consider this for instance: SELECT * FROM A INNER JOIN B ON A.id = B.id WHERE A.x=123. 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. Shouldn't the query planner be smart enough to know that the first query is the same as the second and third? 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. Use a CROSS JOIN instead of an INNER JOIN and the next developer … 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. To me JOIN is strictly a term from Relational Algebra, Relational Model, SQL, you name it. That said, I also agree with him about 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. Being one of the dinosaurs who prefer comma syntax I’d like to say few words on that. In addition to these points, the type of join used in a query implies intent. 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. 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. 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???? Hi there, I am using a LEFT JOIN and i was wondering if having many condition – ie. In my eyes “A JOIN B” and “A,B” are nothing more than notations in particular language and both notations are equivalent. The answer is: It depends! Want to get weekly updates listing the latest blog posts? (Inner, outer, natural .. who cares? Though subqueries have unique abilities, there are times when it is better to use other SQL constructs such as joins. Virtually any expression that would work in a WHERE clause is okay for an ON clause. The user was comparing 3 queries and wondered why the first took significantly longer than the other 2. 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