union

Why union static members not stored as a union?

由 痞子三分冷 提交于 2020-02-15 10:45:12
问题 In C++ union can contain static members which, as in the case of classes, belong to a class and therefore are common to all objects. union U { long l; int i; static long sl; static int si; }; int U::si; long U::sl; It would be logical to expect that all the union static members stored at the same address similar to non-static members storing. But it is not. A simple example shows that static members are stored under different addresses and can contain independent values. int main() { U u; u

Why union static members not stored as a union?

由 狂风中的少年 提交于 2020-02-15 10:44:26
问题 In C++ union can contain static members which, as in the case of classes, belong to a class and therefore are common to all objects. union U { long l; int i; static long sl; static int si; }; int U::si; long U::sl; It would be logical to expect that all the union static members stored at the same address similar to non-static members storing. But it is not. A simple example shows that static members are stored under different addresses and can contain independent values. int main() { U u; u

MySQL View containing UNION does not optimize well…In other words SLOW!

由 两盒软妹~` 提交于 2020-02-04 01:08:29
问题 I have a view containing a UNION ALL. For example: CRATE VIEW myView as (SELECT col1, col2, col3 FROM tab1) UNION ALL (SELECT col1, col2, col3 FROM tab2) These are large tables containing 10s of millions of rows each. If I write: SELECT * FROM myView LIMIT 1; instead of being immediate, it basically never returns as do other queries written against this view. If I use the LIMIT in a query against the individual underlying tables, it is immediate. I have indexes on the underlying tables. It

2020-01-15 Vba Union用法

由 不问归期 提交于 2020-01-31 23:30:17
当要复制EXCEL多行的时候，可以使用Union dim copyRange as range set copyRange = nothing if copyRange is nothing then set copyRange = sht.range(i & ":" i) else set copyRange = union(copyRange,sht.range(i & ":" i)) end if 来源： CSDN 作者： mystert 链接： https://blog.csdn.net/Q215046120/article/details/104126180

Combining Queries with Set & Join Operators【SQL】

由 核能气质少年 提交于 2020-01-31 00:14:52
SET Operator PROC SQL can combine the results of two or more queries in various ways by using the following set operators: UNION produces all unique rows from both queries. o All unique rows from both tables are selected. o Resulting columns are determined by the first table. o Columns are overlaid in the order they appear, not by matching names. o Overlaid columns must have the same data type. EXCEPT produces rows that are part of the first query only. o Unique rows from the first table that are not found in the second table are selected. o Resulting columns are determined by the first table.

mysql中EXPLAIN 的作用

由 落爺英雄遲暮 提交于 2020-01-30 18:53:50
(一)id列： (1)、id 相同执行顺序由上到下 mysql> explain -> SELECT*FROM tb_order tb1 -> LEFT JOIN tb_product tb2 ON tb1.tb_product_id = tb2.id -> LEFT JOIN tb_user tb3 ON tb1.tb_user_id = tb3.id; +----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-------+--------+---------------+---------+---------+---------------------------+------+-------+ | 1 | SIMPLE | tb1 | ALL | NULL | NULL | NULL | NULL | 1 | NULL | | 1 | SIMPLE | tb2 | eq_ref |

UNION优化limit查询

由 情到浓时终转凉″ 提交于 2020-01-28 13:55:14
mysql> explain -> (select first_name,last_name from sakila.actor order by last_name) -> union all -> (select first_name,last_name from sakila.customer order by last_name) -> limit 20; +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+----------+------------+------+---------------+------+---------+------+------+----------+-------+ | 1 | PRIMARY | actor | NULL | ALL | NULL | NULL | NULL | NULL |

MYSQL调优实战

由 ╄→гoц情女王★ 提交于 2020-01-25 18:33:41
一：基础数据准备 DROP TABLE IF EXISTS `tbl_user`; CREATE TABLE `tbl_user` ( `id` int ( 11 ) NOT NULL AUTO_INCREMENT, `username` varchar( 255 ) DEFAULT NULL, `email` varchar( 20 ) DEFAULT NULL, `age` tinyint( 4 ) DEFAULT NULL, `type` int ( 11 ) DEFAULT NULL, `create_time` datetime DEFAULT NULL, PRIMARY KEY (`id`) ) ENGINE =InnoDB AUTO_INCREMENT= 10 DEFAULT CHARSET= utf8; INSERT INTO `tbl_user` VALUES ( ' 1 ' , ' admin ' , ' admin@126.com ' , ' 18 ' , ' 1 ' , ' 2018-07-09 11:08:57 ' ), ( ' 2 ' , ' mengday ' , ' mengday@163.com ' , ' 31 ' , ' 2 ' , ' 2018-07-09 11:09:00 ' ), ( ' 3 ' , ' mengdee ' , '

selecting values from two tables using union but with different key

由 寵の児 提交于 2020-01-25 12:56:40
问题 I have two tables: teacher-record and student-record. teacher-record has two columns: teacher_id, email (email of teacher) student-record has two columns: student_id, email (email of student) I am trying to select teacher_id and student_id using single query by using a union. This is the MySQL query I used (I am also using codeignitor). public function get_std_tech_id($std_email, $tech_email) { $query = $this->db->query(" SELECT `student_id` AS std FROM `student-record` WHERE `email`= '$std

selecting values from two tables using union but with different key

由 浪尽此生 提交于 2020-01-25 12:55:08
问题 I have two tables: teacher-record and student-record. teacher-record has two columns: teacher_id, email (email of teacher) student-record has two columns: student_id, email (email of student) I am trying to select teacher_id and student_id using single query by using a union. This is the MySQL query I used (I am also using codeignitor). public function get_std_tech_id($std_email, $tech_email) { $query = $this->db->query(" SELECT `student_id` AS std FROM `student-record` WHERE `email`= '$std
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