8.3.5 Multiple-Column Indexes MySQL can create composite indexes (that is, indexes on multiple columns). An index may consist of up to 16 columns. For certain data types, you can index a prefix of the column (seeSection 8.3.4, “Column Indexes”). ...
比如INDEX idx_book_id_hero_name (book_id, hero_name) USING BTREE,即对book_id, hero_name两列建立了一个联合索引。 A multiple-column index can be considered a sorted array, the rows of which contain values that are created by concatenating the values of the indexed columns. 联合索引是多列...
They are not useful if the lookup does not start from the leftmost side of the indexed columns. For example, this index won’t help you find all people named Bill or all people born on a certain date, because those columns are not leftmost in the index. Likewise, you can’t use the ...
创建索引 CREATE [ UNIQUE | FULLTEXT ] INDEX index_name ON table_name ( index_col_name,... ) ; 2). 查看索引 SHOW INDEX FROM table_name ; 3). 删除索引 DROP INDEX index_name ON table_name ; 案例演示: 先来创建一张表 tb_user,并且查询测试数据。 create table tb_user( id int primary...
1可以由字母、数字、下划线、@、#、＄2区分大小写3唯一性4不能使用关键字如 create select5不能单独使用数字6最长128位 三)数据库相关操作 1、查看数据库 1show databases;2show create database db1;3select database(); 2、选择数据库 use databasename; ...
多列索引(Multiple-Column Index,又称联合索引、复合索引)顾名思义就是几个列共同组成一个索引。多列索引最多由16个列组成。多列索引遵守最左前缀原则。 最左前缀原则就是在查询数据时,以最左边的列为基准进行索引匹配。例如,有个索引mul_index(col1, col2, col3),在进行索引查询的时候,只有(col1)、(col...
生成批量修改 char 为 varchar2select 'alter table '||a.TABLE_NAME||' modify '||a.COLUMN_NAME||' VARCHAR2('||data_length||');' from all_tab_columns a where a.data_type='CHAR' AND OWNER='用户名'; 通过rtrim 函数把数据右边的空格清除掉 update 表名set 列名1=rtrim(列名1); mysql...
CREATE INDEX cannot be used to create a PRIMARY KEY; use ALTER TABLE instead. For more information about indexes, see Section 10.3.1, “How MySQL Uses Indexes”. InnoDB supports secondary indexes on virtual columns. For more information, see Section 15.1.20.9, “Secondary Indexes and ...
COLUMNS:存储列定义详情 STATISTICS:索引使用情况 Performance Schema 专用于性能监控(5.5+) 动态收集服务器运行指标 关键表举例: events_waits_current:当前等待事件 memory_usage:内存分配详情 系统数据库 mysql:用户权限、时区信息 sys:高级性能视图(5.7+)
The error message “MySQL 1503: A Unique Index Must” is usually encountered when trying to create a unique index on a column or set of columns that contains duplicate values. A unique index ensures that each value in the indexed column(s) is unique, allowing efficient retrieval and preventin...