join | 易学教程

Join tables in two databases using SQLAlchemy

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问题 I am working with two MySQL Databases. I want to join a table from DB 1 with a table from DB2 in SQLAlchemy. I am using automap_base while creating data access layer in sqlalchemy as follows... class DBHandleBase(object): def __init__(self, connection_string='mysql+pymysql://root:xxxxxxx@localhost/services', pool_recycle=3600): self.Base_ = automap_base() self.engine_ = create_engine(connection_string, pool_recycle = pool_recycle) self.Base_.prepare(self.engine_, reflect=True) self.session_ =

HIVE中join、semi join、outer join举例详解

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转自 http://www.cnblogs.com/xd502djj/archive/2013/01/18/2866662.html 举例子： hive> select * from zz0; 111111 222222 888888 hive> select * from zz1; 111111 333333 444444 888888 hive> select * from zz0 join zz1 on zz0.uid = zz1.uid; 111111 111111 888888 888888 hive> select * from zz0 left outer join zz1 on zz0.uid = zz1.uid; 111111 111111 222222 NULL 888888 888888 hive> select * from zz0 right outer join zz1 on zz0.uid = zz1.uid; NULL 111111 111111 NULL 333333 NULL 444444 888888 888888 hive> select * from zz0 full outer join zz1 on zz0.uid = zz1.uid; NULL 111111 111111 222222 NULL NULL 333333 NULL

Is there a “not equal” in a linq join

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问题 I am trying accomplish the LINQ query below but I need a "not equal" instead of equal, so that filteredEmployees has all employees from groupA minus groupB. List<Employee> groupA = getEmployeeA(); List<Employee> groupB = getEmployeeB(); var filteredEmployees = from a in groupA join b in groupB on a.Name equals b.Name select a; 回答1: You don't need a join for that: var filteredEmployees = groupA.Except(groupB); Note that this will be a sequence of unique employees - so if there are any

SQL Filter criteria in join criteria or where clause which is more efficient

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问题 I have a relatively simple query joining two tables. The "Where" criteria can be expressed either in the join criteria or as a where clause. I'm wondering which is more efficient. Query is to find max sales for a salesman from the beginning of time until they were promoted. Case 1 select salesman.salesmanid, max(sales.quantity) from salesman inner join sales on salesman.salesmanid =sales.salesmanid and sales.salesdate < salesman.promotiondate group by salesman.salesmanid Case 2 select

MySql Count cannot show 0 values

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问题 I have two tables one Employee and mailing Subscriptions Employee looks like this: Name (pk) | Surname | Age mailing Subsriptions MailId (pk)| EmployeeName (fk)|Description | Date I wanted to subscription number for each customer, therefore I tried the following query: Select COUNT(c.Name) From Employee INNER JOIN mailingSubscriptions as m ON c.Name = m.EmployeeName; It will give me all counts for each Employee that has an entry in the mailing subscription. My problem is that I want to see

34 到底可不可以使用join？

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34 到底可不可以使用 join ？在实际生产中，关于 join 语句使用的问题，一般会集中在以下两类： --1 dba 不让使用 join ，使用 join 有什么问题呢 --2 如果有两个大小不同的表做 join ，应该用哪个表做驱动表呢？创建 2 个表作为测试 CREATE TABLE `t34` ( `id` int(11) NOT NULL, `a` int(11) DEFAULT NULL, `b` int(11) DEFAULT NULL, PRIMARY KEY (`id`), KEY `a` (`a`) ) ENGINE=InnoDB; drop procedure idata_t34; delimiter ;; create procedure idata_t34() begin declare i int; set i=1; while(i<=1000)do insert into t34 values(i, i, i); set i=i+1; end while; end;; delimiter ; call idata_t34(); create table t35 like t34; insert into t35 (select * from t34 where id<=100) commit; select * from t34; select

why we have need of left and right join

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问题 I have been asked in an interview that why we need left and right join separately while we can get same result by using any of them by changing the order of table? Please help me. 回答1: If you only have 2 tables to join, then yes , you will not see the difference at all. But for 3 or more tables, the order of the tables being joined matters. Check this question to see examples of it. 回答2: Technically, the language doesn't need both. In fact, I avoid right join and see no issues at all. Left

why we have need of left and right join

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How to add GROUP_CONCAT to LEFT JOIN query?

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问题 I have this query (and results): select articles.article_id, articles.article_text, article_photos.photo_filename from articles left join article_photos on article_photos.article_id=articles.article_id >>> results 1,some_text,photo1.jpg 1,some_text,photo2.jpg 1,some_text,photo3.jpg How do I incorporate GROUP_CONCAT to this so that I get: >>> results 1,some_text,photo1.jpg NULL,NULL,photo2.jpg NULL,NULL,photo3.jpg Basically, I have a table with articles, and related table with images. An

How to add GROUP_CONCAT to LEFT JOIN query?

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