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数据库提出了一些规范,这些规范被称为范式(Normal Form) 目前有迹可寻的共有8种范式,一般需要3范式即可 ◆ 第一范式(1NF):强调的是列的原子,即列不能够再分成其他几列。考虑这样一个表:【联系人】(姓名,性别电话) 如果在实际场景中,一个联系人有家庭电话和公司电话,那么这种表结构设计就没有达到 1NF。
nnnnnn(By default, relay log file names have the form host_name-relay-bin.nnnnnn in the data directory); relay_log_index = relay-log.index #relay-log的索引文件名 log-warnings = 1 # 将警告打印输出到错误 log 文件.如果你对于MySQL有任何问题,你应该打开警告 log 并且仔细审查错误日志,查出可能...
(Any memory configured as IndexMemory is automatically added to that used by DataMemory to form a common resource pool.) The memory space allocated by DataMemory consists of 32KB pages, which are allocated to table fragments. Each table is normally partitioned into the same number of ...
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• Error number: 1031; Symbol: ER_ILLEGAL_HA; SQLSTATE: HY000 Message: Table storage engine for '%s' doesn't have this option • Error number: 1032; Symbol: ER_KEY_NOT_FOUND; SQLSTATE: HY000 Message: Can't find record in '%s' • Error number: 1033; Symbol: ER_NOT_FORM_FILE...
(Any memory configured as IndexMemory is automatically added to that used by DataMemory to form a common resource pool.) The memory space allocated by DataMemory consists of 32KB pages, which are allocated to table fragments. Each table is normally partitioned into the same number of ...
select * form student as s left join class as c on s.cls_id=c.cls_id Having is null; 1. 2. 自关联:常用多个等级管理关系,如省市县 将网上下载的sql文件插入到现有的表格中:source 表.sql --创建china 表格,包含中国所有的省和市。
隐式内链接:form子句 后面直接写 多个表名 使用where指定连接条件的 这种连接方式是 隐式内连接 语法格式: SELECT 字段名 FROM 左表, 右表 WHERE 连接条件; 示例: 需求1:查询所有商品信息和对应的分类信息 # 隐式内连接 SELECT * FROM products,category WHERE category_id = cid; 需求2:需求查询商品表的商...
MySQL scans thephone_numberindex to find the entry for555-7271, which contains the primary key entryZawodnybecausephone_book’s primary index is the last name. MySQL then skips to the relevant entry in the database itself. In other words, lookups based on your primary key happen exceedingly ...