union | 易学教程

join columns separated by delimiter in same table

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问题 I have the following data set color_code fav_color_code color_code_name fav_color_name 1|2 5 blue|white black 3|4 7|9 green|red pink|yellow I need to join first value of color_code to first value of color_code_name and second value of color_code to second value of color_code_name etc.. code color 1 blue 2 white 5 black 3 green 4 red 7 pink 9 yellow I am using the below code but it is doing cross join since I dont have id to join upon. This code work if I am mapping 2 columns but not multiple

Avro Schema. How to set type to “record” and “null” at once

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问题 I need to mix "record" type with null type in Schema. "name":"specShape", "type":{ "type":"record", "name":"noSpecShape", "fields":[ { "name":"bpSsc", "type":"null", "default":null, "doc":"SampleValue: null" },... For example, for some datas specShape may be null. So if I set type to "name":"specShape", "type":{ **"type":["record", "null"],** "name":"noSpecShape", "fields":[ { "name":"bpSsc", "type":"null", "default":null, "doc":"SampleValue: null" },... it says No type: {"type":["record",

Avro Schema. How to set type to “record” and “null” at once

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Avro Schema. How to set type to “record” and “null” at once

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MySQL 查询优化器（二）

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1.6多个查询字段（常量条件）多个查询字段的查询处理逻辑如下所示： JOIN:prepare阶段 setup_tables()：同1.1测试。 setup_fields()：同1.1测试。 setup_conds()：同1.4测试。 JOIN:optimize阶段 optimize_cond()：类似1.4测试，不同之处在于查询的where条件中有恒等常量，在优化过程中会调用remove_eq_conds将1=1条件删除。 make_join_statistics()：与1.4测试类似，由于where条件查询有两个，并且其中一个条件可以通过索引查询。因此首先通过调用update_ref_and_keys()（sql\sql_select.cc:3967）函数，查找可以使用索引的字段。 SQL_SELECT::test_quick_select()：同1.5索引测试 get_key_scans_params()：同1.5索引测试。 choose_plan()：同1.3测试。 greedy_search()：同1.3测试。 best_extension_by_limited_search()：同1.5索引测试。 JOIN:exec阶段以下操作同1.3测试。通过测试可以看出，对于常量等式对查询优化器来说没有任何意义，会在optimize_conds时将常量等式删除

SQL查询中的UNION ALL和UNION区别

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UNION和UNION ALL的重要的区别关于对重复结果的处理。 UNION 在合并子查询重复的记录只保留一条，而 UNION All 并不合并子查询的重复记录。现举例说明它们之间的区别。示例1：查询职位为‘CLERK’员工信息。 SQL> SELECT EMPNO,ENAME,JOB DEPTNO FROM EMP WHERE JOB='CLERK'; EMPNO ENAME DEPTNO ---------- ---------- --------- 7369 SMITH CLERK 7876 ADAMS CLERK 7900 JAMES CLERK 7934 MILLER CLERK 示例2：查询部门编号为20员工信息。 SQL> SELECT EMPNO,ENAME,JOB DEPTNO FROM EMP WHERE DEPTNO=20; EMPNO ENAME DEPTNO ---------- ---------- --------- 7369 SMITH CLERK 7566 JONES MANAGER 7788 SCOTT ANALYST 7876 ADAMS CLERK 7902 FORD ANALYST 示例3：使用 UNION ALL 合并示例1和示例2的子查询。 SQL> SELECT EMPNO,ENAME,JOB DEPTNO FROM EMP

Oracle中的Union、Union All、Intersect、Minus

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众所周知的几个结果集集合操作命令，今天详细地测试了一下，发现一些问题，记录备考。假设我们有一个表Student，包括以下字段与数据： drop table student; create table student ( id int primary key, name nvarchar2(50) not null, score number not null ); insert into student values(1,'Aaron',78); insert into student values(2,'Bill',76); insert into student values(3,'Cindy',89); insert into student values(4,'Damon',90); insert into student values(5,'Ella',73); insert into student values(6,'Frado',61); insert into student values(7,'Gill',99); insert into student values(8,'Hellen',56); insert into student values(9,'Ivan',93); insert into student values(10,'Jay',90);

UNION和UNION ALL有什么区别？

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问题： UNION 和 UNION ALL 什么区别？解决方案：参考一： https://stackoom.com/question/CzF/UNION和UNION-ALL有什么区别参考二： https://oldbug.net/q/CzF/What-is-the-difference-between-UNION-and-UNION-ALL 来源： oschina 链接： https://my.oschina.net/u/3797416/blog/3220062

Do pointers inside a union share same memory location?

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问题 I was going through an article to learn about union and I had understood that the size of a Union depends upon the largest variable size and the variables share the same memory. So the concept was pretty clear for me but in the article author said that using "union" for a binary tree was worthwhile when it had two pointers to point other two child. A question arose in my mind for the "What are applications of union?" section of that article, what would be the possible explanations for

SQL UNION

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SQL UNION 操作符 UNION 操作符用于合并两个或多个 SELECT 语句的结果集。请注意，UNION 内部的 SELECT 语句必须拥有相同数量的列。列也必须拥有相似的数据类型。同时，每条 SELECT 语句中的列的顺序必须相同。 SQL UNION 语法 SELECT column_name(s) FROM table_name1 UNION SELECT column_name(s) FROM table_name2 注释：默认地，UNION 操作符选取不同的值。如果允许重复的值，请使用 UNION ALL。 SQL UNION ALL 语法 SELECT column_name(s) FROM table_name1 UNION ALL SELECT column_name(s) FROM table_name2 另外，UNION 结果集中的列名总是等于 UNION 中第一个 SELECT 语句中的列名。下面的例子中使用的原始表： Employees_China: E_ID E_Name 01 Zhang, Hua 02 Wang, Wei 03 Carter, Thomas 04 Yang, Ming Employees_USA: E_ID E_Name 01 Adams, John 02 Bush, George 03 Carter, Thomas 04