union

Are PostgreSQL VIEWS created newly each time they are queried against?

由 冷暖自知 提交于 2020-01-02 00:59:24
问题 I am creating a web app which has some complex underlying associations. In order to solve several issues I was having I created a UNION View. There are probably a lot of other ways this could be solved. But I am now considering the efficiency of my design, and I wanted to know if a VIEW is newly created each time it is queried, or is it only created once, and kept updated. To elaborate, if I have table_a (100 records) and table_b (100 records) and make a UNION View, then I have created a view

union all的问题报错及解决办法

由 本秂侑毒 提交于 2019-12-31 12:47:25
union all的问题报错及解决办法 union all的用法，其实不陌生的。union all上下的字段名要一致，字段数据类型要一致。但是满足上面两点后还是报错。 例如： select a, b from t1 union all select a,b from t2 上面的语句会报错，不知道为啥。 后面发现，原来是hive不支持顶层查询，换句话说，换成下面的代码就可以了 select a, b from ( select a,b from t1 union all select a,b from t2 ) a 来源： CSDN 作者： alicheng 链接： https://blog.csdn.net/alicheng/article/details/103770144

mysql优化之 EXPLAIN（一）

由 旧街凉风 提交于 2019-12-30 01:48:23
数据库优化最常用的命令就是用explain查看一下写的sql是否用到了索引: 如： (root@localhost) [akapp]>explain select * from sc_activity where id='3a2cd2a83892d322c1332acdfe'; +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+------+---------------+------+---------+------+------+-----------------------------------------------------+ | 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Impossible WHERE noticed after

How can I get the intersection, union, and subset of arrays in Ruby?

由 二次信任 提交于 2019-12-29 10:07:17
问题 I want to create different methods for a class called Multiset. I have all the required methods, but I'm unsure of how to write intersection, union, and subset methods. For intersection and union, my code starts like this: def intersect(var) x = Multiset.new end Here is an example: X = [1, 1, 2, 4] Y = [1, 2, 2, 2] then the intersection of X and Y is [1, 2] . 回答1: Utilizing the fact that you can do set operations on arrays by doing & (intersection), - (difference), and | (union). Obviously I

How to combine 2different IQueryable/List/Collection with same base class? LINQ Union and Covariance issues

由 China☆狼群 提交于 2019-12-29 01:34:14
问题 I am trying to combine (union or concat) two lists/collection into one. The two lists have a common base class. e.g. I've tried this: IQueryable<ContractItem> contractItems = myRepository.RetrieveContractItems(); IQueryable<ChangeOrderItem> changeOrderItems = myRepository.RetrieveChangeOrderItems(); IQueryable<ItemBase> folderItems = contractItems.Concat<ItemBase>(changeOrderItems); But am getting the LINQ error DbUnionAllExpression requires arguments with compatible collection ResultTypes.

Merge two rows in SQL

由 僤鯓⒐⒋嵵緔 提交于 2019-12-28 03:45:10
问题 Assuming I have a table containing the following information: FK | Field1 | Field2 ===================== 3 | ABC | *NULL* 3 | *NULL* | DEF is there a way I can perform a select on the table to get the following FK | Field1 | Field2 ===================== 3 | ABC | DEF Thanks Edit: Fix field2 name for clarity 回答1: Aggregate functions may help you out here. Aggregate functions ignore NULLs (at least that's true on SQL Server, Oracle, and Jet/Access), so you could use a query like this (tested on

UNION query with codeigniter's active record pattern

由 元气小坏坏 提交于 2019-12-27 11:06:38
问题 How to do UNION query with PHP CodeIgniter framework's active record query format? 回答1: CodeIgniter's ActiveRecord doesn't support UNION, so you would just write your query and use the ActiveRecord's query method. $this->db->query('SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2'); 回答2: This is a quick and dirty method I once used // Query #1 $this->db->select('title, content, date'); $this->db->from('mytable1'); $query1 = $this->db->get()->result(); //

UNION query with codeigniter's active record pattern

由 六眼飞鱼酱① 提交于 2019-12-27 11:06:23
问题 How to do UNION query with PHP CodeIgniter framework's active record query format? 回答1: CodeIgniter's ActiveRecord doesn't support UNION, so you would just write your query and use the ActiveRecord's query method. $this->db->query('SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2'); 回答2: This is a quick and dirty method I once used // Query #1 $this->db->select('title, content, date'); $this->db->from('mytable1'); $query1 = $this->db->get()->result(); //

SQL查出的字段一列分成两列

由 久未见 提交于 2019-12-27 01:58:16
CREATE TABLE #tb( lh VARCHAR(10)) INSERT INTO #tb SELECT '12-1-2202' union all SELECT '12-1-2301' union all SELECT '12-1-2302' union all SELECT '12-1-2401' union all SELECT '12-1-2402' union all SELECT '12-1-2501' union all SELECT '12-1-2502' union all SELECT '12-1-2601' union all SELECT '12-1-2602' union all SELECT '12-2-101' union all SELECT '12-2-102' union all SELECT '12-2-201' union all SELECT '12-2-202' union all SELECT '12-2-301' union all SELECT '12-2-302' union all SELECT '12-2-401' union all SELECT '12-2-402' SELECT lh,SUBSTRING(lh,4,1) as a INTO #s from #tb select a.lh as ydy,b.lh

SQL UNION and ORDER BY

由 怎甘沉沦 提交于 2019-12-25 14:10:15
问题 I have an annoying SQL statement that seem simple but it looks awfull. I want the sql to return a resultset with userdata ordered so that a certain user is the first row in the resultset if that users emailaddress is in the companies table. I have this SQL that returns what i want but i think it looks awful: select 1 as o, * from Users u where companyid = 1 and email = (select email from companies where id=1) union select 2 as o, * from Users u where companyid = 1 and email <> (select email
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