inner-join | 易学教程

what is the difference between join keyword and inner join keyword in oracle sql? [duplicate]

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问题 This question already has answers here : Difference between JOIN and INNER JOIN (6 answers) Closed 6 years ago . I can't find documentations on the key word join but I saw examples on the web using it. I was doing some experiment with it in Oracle hr schema, where I have table departments : deparment_name manager_id location_id A table employees : first_name employee_id And table locations : location_id city Query should return the department_name, first_name of the manager of the department,

Configure Map Side join for multiple mappers in Hadoop Map/Reduce

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问题 I have a question about configuring Map/Side inner join for multiple mappers in Hadoop. Suppose I have two very large data sets A and B, I use the same partition and sort algorithm to split them into smaller parts. For A, assume I have a(1) to a(10), and for B I have b(1) to b(10). It is assured that a(1) and b(1) contain the same keys, a(2) and b(2) have the same keys, and so on. I would like to setup 10 mappers, specifically, mapper(1) to mapper(10). To my understanding, Map/Side join is a

MySQL INNER JOIN select only one row from second table

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I have a users table and a payments table, for each user, those of which have payments, may have multiple associated payments in the payments table. I would like to select all users who have payments, but only select their latest payment. I'm trying this SQL but i've never tried nested SQL statements before so I want to know what i'm doing wrong. Appreciate the help SELECT u.* FROM users AS u INNER JOIN ( SELECT p.* FROM payments AS p ORDER BY date DESC LIMIT 1 ) ON p.user_id = u.id WHERE u.package = 1 John Woo You need to have a subquery to get their latest date per user ID . SELECT a.*, c.*

inner join and where in() clause performance?

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I can get same result for these queries, but which one is the fastest, and most efficient? where in() or inner join? SELECT `stats`.`userid`,`stats`.`sumpoint` FROM `stats` INNER JOIN users ON `stats`.`userid` = `users`.`userid` WHERE `users`.`nick` = '$nick' ORDER BY `statoylar`.`sumpoint` DESC limit 0,10 and SELECT `stats`.`userid`,`stats`.`sumpoint` FROM `stats` WHERE userid IN ( SELECT userid FROM `users` WHERE `users`.`nick` = '$nick' ) ORDER BY `stats`.`sumpoint` DESC limit 0,10 Depends on your SQL engine. Newer SQL systems that have reasonable query optimizers will most likely rewrite

SQL DELETE with INNER JOIN

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There are 2 tables, spawnlist and npc , and I need to delete data from spawnlsit . npc_templateid = n.idTemplate is the only thing that "connect" the tables. I have tried this script but it doesn't work. I have tried this: DELETE s FROM spawnlist s INNER JOIN npc n ON s.npc_templateid = n.idTemplate WHERE (n.type = "monster"); Add .* to s in your first line. Try: DELETE s.* FROM spawnlist s INNER JOIN npc n ON s.npc_templateid = n.idTemplate WHERE (n.type = "monster"); If the database is InnoDB then it might be a better idea to use foreign keys and cascade on delete, this would do what you

Conditions in LEFT JOIN (OUTER JOIN) vs INNER JOIN

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问题 SELECT A.COL1, B.COL1,C.COL1 FROM TABLEA A LEFT JOIN TABLEB B ON A.COL1 = B.COL1 LEFT JOIN TABLEC C ON ( C.COL3 IS NOT NULL AND ( C.COL2 = 664 AND A.COL1 = C.COL1 ) ) In regards to technicalities of SQL, what does the condition written in parentheses after LEFT JOIN TABLE C ON mean? Why are those necessary? 回答1: An inner join (JOIN or INNER JOIN, CROSS JOIN or comma) first does a CROSS JOIN. (Ie returns all rows that can be made by appending a row from its left table and a row from its right

How do I convert multiple inner joins in SQL to LINQ?

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I've got the basics of LINQ-to-SQL down, but I've been struggling trying to get JOINs to work properly. I'd like to know how to convert the following to LINQ-to-SQL (ideally using method chaining, as that is my preferred format). SELECT c.CompanyId, c.CompanyName, p.FirstName + ' ' + p.LastName as AccountCoordinator, p2.FirstName + ' ' + p2.LastName as AccountManager FROM dbo.Companies c INNER JOIN dbo.Persons p ON c.AccountCoordinatorPersonId = p.PersonId INNER JOIN dbo.Persons p2 ON c.AccountManagerPersonId = p2.PersonId Using query syntax: from c in dbo.Companies join p in dbo.Persons on c

Rails ActiveRecord :joins with LEFT JOIN instead of INNER JOIN

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I have this code User.find(:all, :limit => 10, :joins => :user_points, :select => "users.*, count(user_points.id)", :group => "user_points.user_id") which generates following sql SELECT users.*, count(user_points.id) FROM `users` INNER JOIN `user_points` ON user_points.user_id = users.id GROUP BY user_points.user_id LIMIT 10 is it possible to make LEFT JOIN instead of INNER JOIN other way than User.find_by_sql and manualy typing the query? Kylo You can try this User.find(:all, limit: 10, joins: "LEFT JOIN `user_points` ON user_points.user_id = users.id" , select: "users.*, count(user_points.id

using where and inner join in mysql

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问题 I have three tables. locations ID | NAME | TYPE | 1 | add1 | stat | 2 | add2 | coun | 3 | add3 | coun | 4 | add4 | coun | 5 | add5 | stat | schools ID | NAME 1 | sch1 2 | sch2 3 |sch3 school_locations ID |LOCATIONS_ID |SCHOOL_ID 1 | 1 |1 2 | 2 |2 3 | 3 |3 Here the table locations contains all the locations of the application.Locations for school are called by ID's. when i use the query select locations.name from locations where type="coun"; it displays names with type "coun" But I want to

MySQL: Inner join vs Where [duplicate]

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This question already has an answer here: Explicit vs implicit SQL joins 12 answers Is there a difference in performance (in mysql) between Select * from Table1 T1 Inner Join Table2 T2 On T1.ID = T2.ID And Select * from Table1 T1, Table2 T2 Where T1.ID = T2.ID ? As pulled from the accepted answer in question 44917: Performance wise, they are exactly the same (at least in SQL Server) but be aware that they are deprecating the implicit outer join syntax. In MySql the results are the same. I would personally stick with joining tables explicitly... that is the "socialy acceptable" way of doing it.