create database crashcourse default character set utf8mb4 collate utf8mb4_general_ci; # 创建指定的数据库(指定字符集和排序规则) use crashcourse; # 指定使用的数据库 show databases; # 了解数据库,返回数据库列表 show tables; # 返回数据库内表的列表 show columns from customers; # 查看customers表...
to use index hints to pass along advice about which lookup method is more efficientfora particular query.Cardinality can also apply to the numberofdistinct values presentinmultiple columns,asina composite index.See Also column,composite index,index,index hint,persistent statistics,random dive,selectivity...
You may use a subquery that returns multiple columns, if the purpose is row comparison. In other contexts, the subquery must be a scalar operand. See Section 15.2.15.5, “Row Subqueries”. Incorrect number of rows from subquery: ERROR 1242 (ER_SUBSELECT_NO_1_ROW) SQLSTATE = 21000 Mess...
unique_subquery unique_subquery 是针对在一些包含IN子查询的查询语句中,如果查询优化器决定将IN子查询转换为EXISTS子查询,而且子查询可以使用到主键进行等值匹配的话,那么该子查询执行计划的type列的值就是unique_subquery。 range 如果使用索引获取某些~范围区间`的记录,那么就可能使用到`range`访问方法 index 当我们...
• : The row refers to the result of a materialized subquery for the row with an id value of N. partitions The partitions from which records would be matched by the query. The value is NULL for nonpartitioned tables. possible_keys ...
Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite index. See Also column, composite index, index, index hint, persistent statistics, random dive, selectivity, unique constraint. 1.
Cardinality can also apply to the number of distinct values present in multiple columns, as in a composite index. See Also column, composite index, index, index hint, persistent statistics, random dive, selectivity, unique constraint. 如何让sql强制使用索引...
I tried using a UNION ALL as a subquery and then a group by but that was rather slow (1.8 seconds). In the end I ended up adding the amount the primary key divided by pi (something which had enough decimal places that I could be fairly certain my data would never have. The Amount...
You may use a subquery that returns multiple columns, if the purpose is row comparison. In other contexts, the subquery must be a scalar operand. See Section 13.2.10.5, “Row Subqueries”. Incorrect number of rows from subquery: ERROR 1242 (ER_SUBSELECT_NO_1_ROW) SQLSTATE = 21000 Mess...
我们在MySQL 体系结构中介绍了 MySQL 的服务器逻辑结构,其中查询优化器(optimizer)负责生成 SQL 语句的执行计划,是决定查询性能的一个关键组件。本文将会深入分析 MySQL 优化器工作的原理以及如何控制优化器来实现 SQL 语句的优化。 优化器概述 MySQL 优化器使用基于成本的优化方式(Cost-based Optimization),以 SQL 语...