join | 易学教程

Messages: Query a user's chats

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问题 For an app like iOS Messages: How should I design the server database queries & schema? Queries Create a user. Get a user's chats. Get the messages between two users. Add a message to a chat (creating the chat if it doesn't yet exist). Note : Every chat only has two users (no group chats). Every chat's id equals this unique unordered pairing function applied to user_id & with_id . How should I query a user's chats, each with its latest message? Example User Interface (UI) See my design &

SQL的各种连接Join详解

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SELECT语句执行顺序 SELECT语句中子句的执行顺序与SELECT语句中子句的输入顺序是不一样的，所以并不是从SELECT子句开始执行的，而是按照下面的顺序执行：开始->FROM子句->WHERE子句->GROUP BY子句->HAVING子句->ORDER BY子句->SELECT子句->LIMIT子句->最终结果每个子句执行后都会产生一个中间结果，供接下来的子句使用，如果不存在某个子句，就跳过对比了一下，mysql和sql执行顺序基本是一样的, 标准顺序的 SQL 语句为: select 考生姓名, max(总成绩) as max总成绩 from tb_Grade where 考生姓名 is not null group by 考生姓名 having max(总成绩) > 600 order by max总成绩在上面的示例中 SQL 语句的执行顺序如下: 　　 (1). 首先执行 FROM 子句, 从 tb_Grade 表组装数据源的数据　　 (2). 执行 WHERE 子句, 筛选 tb_Grade 表中所有数据不为 NULL 的数据　　 (3). 执行 GROUP BY 子句, 把 tb_Grade 表按 "学生姓名" 列进行分组(注：这一步开始才可以使用select中的别名，他返回的是一个游标，而不是一个表

java字符串常用方法知识总结

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String类的 substring方法可以从一个较大的字符串提取出一个子串。例如： String greeting = “Hello”; String s = greeting.substring®, 3); 创建了一个由字符“ Hel” 组成的字符串空串与null的区别空串 “” 是长度为 0 的字符串。可以调用以下代码检查一个字符串是否为空： if (str.lengthQ = 0)或 if (str.equals("")) 空串是一个 Java 对象，有自己的串长度（0 ) 和内容（空）。 String 变量还可以存放一个特殊的值，名为 null, 这表示目前没有任何对象与该变量关联要检查一个字符串是否为 null, 要使用以下条件： if (str == null) （换句话说空串就是像一栋毛坯房，里边啥都没有，而null，是连毛坯房都没有） char charAt (int index) 返回给定位置的代码单元（返回指定位置的char型字符）例如：String a = “abcd”; System.out.println(“a.charAt(0)”); int codePointAt(int Index) 返回从给定位置开始的码点。 int offsetByCodePoints(int startlndex, int cpCount) 返回从

Inner join between three tables in mysql

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问题 I have 3 tables called Companies Contacts Campaigns Contacts has foreign key of companies. Companies has foreign key of campaigns. All of them have a column 'name'. I need a join table which will have contact name company name and campaign name. contact id name company_id companies id name campaign_id campaigns id name company_id 回答1: select con.name as contact_name , com.name as company_name,campa.name as campaign_name from contact con inner join company com on con.companyid = com.companyid

How to SQL Select a one to many relation and merge the output

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问题 This has been dramatically updated since i got closer to the solution I guess the title is not the best but I did not know how to explain it better. I have locations with coordinates in two related tables. Table locations(id, name, description, created_at) and locations_coordinates(location_id, lat, lng, coordinates_order) . I am storing an unknown amount of coordinates (a polygon), that`s why I use two tables. Now I am running the following query SELECT l.id, l.name, l.description, l.created

How to SQL Select a one to many relation and merge the output

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MySQL join and exclude?

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问题 I have two tables, table A one with two columns: IP and ID, and table B with columns: ID and extra information. I want to extract the rows in table B for IPs that are not in table A. So if I have a rows in table A with id = 1 ip = 000.000.00 id = 2 ip = 111.111.11 and I have rows in table B id = 1 id = 2 then, given ip = 111.111.11, how can I return row 1 in table B? 回答1: The simplest and most easy-to-read way to spell what you're describing is: SELECT * FROM `B` WHERE `ID` NOT IN (SELECT `ID

Group by week in MySQL

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问题 Table structure is, Table name: btq_user name, email, kall, state_id, datain Now i want to count number records which has kall = 1 or state_id in( 51, 20, 46) by weekly. i need the results only by weekly (Mon - Sun) no matter if year changes. lets say for example 31-12-2012 is Monday and 6-1-2013 is Sunday so result should include that as a week. no matter if year changes it should count only with weeks. This is what i tried but not working. SELECT count( if( A.state_id = 51 or A.state_id =

FQL profile.pic_square and thread.subject

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问题 is it possible to get a join of two tables in the Facebook API, with FQL? The classic way with SQL style don´t work. Hope you can help :) PS: WINNING :) 回答1: As Documentation Says Facebook Query Language, or FQL, enables you to use a SQL-style interface to query the data exposed by the Graph API. It provides for some advanced features not available in the Graph API, including batching multiple queries into a single call. You can execute FQL queries by fetching https://api.facebook.com/method

Hadoop: intervals and JOIN

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问题 I'm very new to Hadoop and I'm currently trying to join two sources of data where the key is an interval (say [date-begin/date-end]). For example: input1: 20091001-20091002 A 20091011-20091104 B 20080111-20091103 C (...) input2: 20090902-20091003 D 20081015-20091204 E 20040011-20050101 F (...) I'd like to find all the records where the key1 overlaps the key2. Is it possible with hadoop ? Where can I find an example of implementation ? Thanks. 回答1: A solution was given on Biostar: http:/