MySQL keys are attributes or columns in a relationaldatabasethat we can use to uniquely identify records in a table. In this tutorial, we’ll look into three primary types of keys inMySQL. These keys are the Primary Key (PRI), Unique Key (UNI), and Multiple Key (MUL). 2. What Are ...
When I specify a FOREIGN KEY constraint, and then do a "desc" on that table, the Foreign Key row displays "MUL" in the Key Column. What is that?Navigate: Previous Message• Next Message Options: Reply• Quote Subject Written By Posted What is "MUL? Tara Wilk March 14, 2008 06...
如果Key 是 UNI, 那么该列是一个唯一值索引的第一列(前导列),并别不能含有空值(NULL); 如果Key 是 MUL, 那么该列的值可以重复, 该列是一个非唯一索引的前导列(第一列)或者是一个唯一性索引的组成部分但是可以含有空值NULL。 如果对于一个列的定义,同时满足上述4种情况的多种,比如一个列既是PRI,又是U...
比如float(9,3),varchar(50)。 COLUMN_KEY:索引类型。可包含的值有PRI,代表主键,UNI,代表唯一键,MUL,可重复。 EXTRA:其他信息。比如主键的auto_increment。 PRIVILEGES:权限。多个权限用逗号隔开,比如 select,insert,update,references COLUMN_COMMENT:字段注释 GENERATION_EXPRESSION:组合字段的公式。 ENGINES 数据库引...
SQL 语句是一种what型语言【想要什么,给你】,语法相对简单易懂SQL (Structured Query Language)语言的划分 DDL(Data Definition Language)- 数据库定义语言;用来定义数据库对象、数据表和列;使用DDL创建、删除、修改数据库的表和结构 DML(Data Manipulation Language)- 数据库操作语言;操作数据库的相关数据,比如增加...
ADD PRIMARY KEY (col3) specifies the action to be performed. PRIMARY KEY (col3) indicates that the "col3" column is being designated as the primary key for the table. This primary key constraint ensures that each value in the "col3" column is unique, serving as the primary identifier ...
File Name What Name Stands For Size Comment Inside File --- --- --- --- ibuf0ibuf.c Insert Buffer / 91,397 Insert buffer include:innodb将头文件(.h,.ic)文件都统一放在这个文件夹下。 lob: lock:Innodb存储引擎锁的实现。 File Name What Name Stands For Size Comment Inside File --- -...
If Key is MUL, the column is the first column of a nonunique index in which multiple occurrences of a given value are permitted within the column. If more than one of the Key values applies to a given column of a table, Key displays the one with the highest priority, in the order...
CAltered_partitions Helper class for encapsulating new/altered partitions during ADD(HASH/KEY)/COALESCE/REORGANIZE PARTITION Cnet::tls::Operation::AnalyzeOperation CAndCombine CMysqlx::Datatypes::Any A helper to allow all field types CJson_scalar_holder::Any_json_scalar Unio...
(扫描全表) possible_keys字段:该列指出Mysql可能会选择使用的索引...key字段:Mysql在执行该条查询语句时,真正选择使用的索引 rows字段:显示MySQL认为它执行查询时必须检查的行数,不是最后得出的结果的真实行数 Extra字段:显示Mysql解析查询时的详细信息...,例如使用了哪一种索引合并优化算法、查询是否使用了临时表...