1. 自然连接 通过mysql 自己的判断完成连接的过程, 而不需要指定连接条件, mysql使用多个表中的相同字段作为连接条件。 内连接 natural join ? inner join using 左外连接 natural left join ? left join using 右外连接 natural right join ? right join
1. 自然连接
-
通过mysql 自己的判断完成连接的过程, 而不需要指定连接条件, mysql使用多个表中的相同字段作为连接条件。
- 内连接 natural join
? inner join using
- 左外连接 natural left join
? left join using
- 右外连接 natural right join
? right join using
ps: left join 和 right join 之间是可以相互转化的
- 内连接 natural join
支持多表连接查询
小结
这个图片, 传上来就是这个样子, 也不知道该怎么旋转, 如果有人知道怎么弄的话, 恳请指教一下。
2. 重定向
- mysql 除了可以将输出结果返回到界面上, 也可以将输出结果写入到文件中。
<code class=" hljs cs"><span class="hljs-keyword">select</span> * <span class="hljs-keyword">into</span> outfile xxx <span class="hljs-keyword">from</span> ....</code>
需要注意的是,他不允许重写已经存在的文件, 但是可以创建新的文件。默认使用’\t’区分字段, 使用’\n’区分记录, 可以修改。
3. 插入数据
- 使用 insert into 插入数据
- 当插入数据冲突的时候, 可以使用on duplicate key update 进行更新数据
- 另外需要注意, 我们可以使用select 查询得到的子句的结果来进行插入
- default
- replace into 用于插入数据, 可以处理主键或者唯一索引冲突问题
- load data infile 可以用来设置从文件中导入数据, 以及数据的格式
4. 删除数据
- delete from xxx limit n
- limit 用来限制记录的数目, 可以与order by 配合使用
- 支持连接删除, 可以用来模拟外键约束
<code class=" hljs livecodeserver"><span class="hljs-built_in">delete</span> <span class="hljs-built_in">from</span> <span class="hljs-constant">one</span>, <span class="hljs-constant">two</span> <span class="hljs-keyword">using</span> <span class="hljs-constant">one</span> join <span class="hljs-constant">two</span> <span class="hljs-command"><span class="hljs-keyword">on</span> <span class="hljs-title">xxx</span> <span class="hljs-title">where</span> <span class="hljs-title">xxx</span></span></code>
5. 清空表
- truncate 直接将表格删除然后新建一个表, 因而, 他的自动增长索引是从0 开始计算的。
- 而 delete from table 数据删除了, 但是他们的自动增长的索引值不不会变为 0 的。
6. 更新表
- replace / insert on duplicate key update
- 条件更新, limit, order by & limit etc
- 支持多个表同时更新
<code class=" hljs sql"><span class="hljs-operator"><span class="hljs-keyword">update</span> [one <span class="hljs-keyword">join</span> two <span class="hljs-keyword">on</span> xxx] <span class="hljs-keyword">set</span> xxx <span class="hljs-keyword">where</span> xxx</span></code>

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