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<updated>2021-05-01T15:01:41.751Z</updated>
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<entry>
<title>2021 5月力扣刷题</title>
<link href="http://aidianfirst.com/2021/05/01/2021%205%E6%9C%88%E5%8A%9B%E6%89%A3%E5%88%B7%E9%A2%98/"/>
<id>http://aidianfirst.com/2021/05/01/2021%205%E6%9C%88%E5%8A%9B%E6%89%A3%E5%88%B7%E9%A2%98/</id>
<published>2021-05-01T14:51:32.000Z</published>
<updated>2021-05-01T15:01:41.751Z</updated>
<content type="html"><![CDATA[<h1 id="五月一日"><a href="#五月一日" class="headerlink" title="五月一日"></a>五月一日</h1><p>今天是力扣690题，员工的重要性。发现用哈希表做的还是不熟，看了一下普通递归思路，这个比较好理解。这段时间看完设计模式和sql，就去看剑指offer，容器还是用少了，不太熟。</p><p>题目：<a href="https://leetcode-cn.com/problems/employee-importance/">https://leetcode-cn.com/problems/employee-importance/</a></p><p>参考：<a href="https://leetcode-cn.com/problems/employee-importance/solution/java-di-gui-by-jonnyhuang-kcyy/">https://leetcode-cn.com/problems/employee-importance/solution/java-di-gui-by-jonnyhuang-kcyy/</a></p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">// Definition for Employee.</span></span><br><span class="line"><span class="comment">class Employee {</span></span><br><span class="line"><span class="comment"> public int id;</span></span><br><span class="line"><span class="comment"> public int importance;</span></span><br><span class="line"><span class="comment"> public List<Integer> subordinates;</span></span><br><span class="line"><span class="comment">};</span></span><br><span class="line"><span class="comment">*/</span></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">Solution</span> </span>{</span><br><span class="line"> <span class="keyword">int</span> sum=<span class="number">0</span>;</span><br><span class="line"> <span class="function"><span class="keyword">public</span> <span class="keyword">int</span> <span class="title">getImportance</span><span class="params">(List<Employee> employees, <span class="keyword">int</span> id)</span> </span>{</span><br><span class="line"> <span class="comment">//foreach遍历list</span></span><br><span class="line"> <span class="keyword">for</span>(Employee e : employees){</span><br><span class="line"> <span class="comment">//找到Employee.id和输入id一样的Employee</span></span><br><span class="line"> <span class="keyword">if</span>(e.id==id){</span><br><span class="line"> sum += e.importance;<span class="comment">//统计重要度</span></span><br><span class="line"> <span class="comment">//对下属遍历</span></span><br><span class="line"> <span class="keyword">for</span>(<span class="keyword">int</span> i : e.subordinates){</span><br><span class="line"> getImportance(employees,i);<span class="comment">//还是执行getImportance进行递归，还是对整个list遍历，所以用employees</span></span><br><span class="line"> } </span><br><span class="line"> }</span><br><span class="line"> }</span><br><span class="line"> <span class="keyword">return</span> sum;</span><br><span class="line"> }</span><br><span class="line">}</span><br></pre></td></tr></table></figure>]]></content>
<summary type="html"><h1 id="五月一日"><a href="#五月一日" class="headerlink" title="五月一日"></a>五月一日</h1><p>今天是力扣690题，员工的重要性。发现用哈希表做的还是不熟，看了一下普通递归思路，这个比较好理解。这段时间看完设计模式和sq</summary>
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<category term="学习" scheme="http://aidianfirst.com/tags/%E5%AD%A6%E4%B9%A0/"/>
<category term="leetcode刷题" scheme="http://aidianfirst.com/tags/leetcode%E5%88%B7%E9%A2%98/"/>
</entry>
<entry>
<title>哈希表应用语句</title>
<link href="http://aidianfirst.com/2021/04/30/%E5%93%88%E5%B8%8C%E8%A1%A8%E5%BA%94%E7%94%A8%E8%AF%AD%E5%8F%A5/"/>
<id>http://aidianfirst.com/2021/04/30/%E5%93%88%E5%B8%8C%E8%A1%A8%E5%BA%94%E7%94%A8%E8%AF%AD%E5%8F%A5/</id>
<published>2021-04-30T09:33:57.000Z</published>
<updated>2021-05-01T14:53:53.200Z</updated>
<content type="html"><![CDATA[<h1 id="哈希表-hashmap"><a href="#哈希表-hashmap" class="headerlink" title="哈希表 hashmap"></a>哈希表 hashmap</h1><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br></pre></td><td class="code"><pre><span class="line">map.put(x, map.getOrDefault(x, <span class="number">0</span>) + <span class="number">1</span>);</span><br><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">value默认从1开始，map每遇到一次x则value值加1，用来统计键出现的次数。</span></span><br><span class="line"><span class="comment">map.getOrDefault(Object key,V defaultValue)</span></span><br><span class="line"><span class="comment">如果在Map中存在key，则返回key所对应的的value。</span></span><br><span class="line"><span class="comment">如果在Map中不存在key，则返回默认值。</span></span><br><span class="line"><span class="comment">*/</span></span><br></pre></td></tr></table></figure>]]></content>
<summary type="html"><h1 id="哈希表-hashmap"><a href="#哈希表-hashmap" class="headerlink" title="哈希表 hashmap"></a>哈希表 hashmap</h1><figure class="highlight java"><table</summary>
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<category term="学习" scheme="http://aidianfirst.com/tags/%E5%AD%A6%E4%B9%A0/"/>
<category term="哈希表" scheme="http://aidianfirst.com/tags/%E5%93%88%E5%B8%8C%E8%A1%A8/"/>
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<entry>
<title>SQL基础</title>
<link href="http://aidianfirst.com/2021/04/27/SQL%E5%9F%BA%E7%A1%80/"/>
<id>http://aidianfirst.com/2021/04/27/SQL%E5%9F%BA%E7%A1%80/</id>
<published>2021-04-27T15:48:31.000Z</published>
<updated>2021-04-30T15:53:32.227Z</updated>
<content type="html"><![CDATA[<h1 id="MYSQL基础"><a href="#MYSQL基础" class="headerlink" title="MYSQL基础"></a>MYSQL基础</h1><pre><code> 数据库·····语言 DDL定义 DML操作（增删改） DQL查询 DCL控制</code></pre><h2 id="1、sql语句创建表"><a href="#1、sql语句创建表" class="headerlink" title="1、sql语句创建表"></a>1、sql语句创建表</h2><pre><code> CREATE创建 DROP删除 ALTER修改 对象：database数据库 table表</code></pre><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">CREATE</span> <span class="keyword">TABLE</span> IF <span class="keyword">NOT</span> <span class="keyword">EXISTS</span> `student`(</span><br><span class="line">`id` <span class="type">INT</span>(<span class="number">5</span>) <span class="keyword">NOT</span> <span class="keyword">NULL</span> AUTO_INCREMENT COMMENT <span class="string">'学号'</span>,</span><br><span class="line">`name` <span class="type">VARCHAR</span>(<span class="number">20</span>) <span class="keyword">NOT</span> <span class="keyword">NULL</span> <span class="keyword">DEFAULT</span> <span class="string">'学生姓名'</span> COMMENT <span class="string">'姓名'</span>,</span><br><span class="line">`password` <span class="type">VARBINARY</span>(<span class="number">30</span>) <span class="keyword">NOT</span> <span class="keyword">NULL</span> <span class="keyword">DEFAULT</span><span class="string">'123456789'</span> COMMENT <span class="string">'密码'</span>,</span><br><span class="line">`sex` <span class="type">VARCHAR</span>(<span class="number">5</span>) <span class="keyword">NOT</span> <span class="keyword">NULL</span> <span class="keyword">DEFAULT</span> <span class="string">'男'</span> COMMENT <span class="string">'性别'</span>,</span><br><span class="line">`age` <span class="type">INT</span>(<span class="number">5</span>) <span class="keyword">NOT</span> <span class="keyword">NULL</span> <span class="keyword">DEFAULT</span> <span class="string">'18'</span> COMMENT <span class="string">'年龄'</span>,</span><br><span class="line">`email` <span class="type">VARCHAR</span>(<span class="number">50</span>) <span class="keyword">DEFAULT</span> <span class="keyword">NULL</span> COMMENT <span class="string">'邮箱'</span>,</span><br><span class="line"><span class="keyword">PRIMARY</span> KEY (`id`)</span><br><span class="line">)ENGINE<span class="operator">=</span>INNODB <span class="keyword">DEFAULT</span> CHARSET<span class="operator">=</span>utf8</span><br><span class="line"></span><br><span class="line">ENGINE 引擎</span><br><span class="line"> <span class="operator">-</span>INNODB现在默认使用，安全性高。</span><br><span class="line"> <span class="operator">-</span>MYISAM早期使用，节约空间。</span><br><span class="line">CHARSET 字符集 一般选择utf8。</span><br><span class="line"></span><br><span class="line"><span class="keyword">SHOW</span> <span class="keyword">CREATE</span> <span class="keyword">TABLE</span> XXX;<span class="comment">-- 查看表的sql创建语句 database对应查看数据库</span></span><br><span class="line"><span class="keyword">DESC</span> XXX;<span class="comment">--查看表的结构</span></span><br><span class="line"></span><br><span class="line"><span class="keyword">ALTER</span> <span class="keyword">TABLE</span> XXX RENAME <span class="keyword">AS</span> ···<span class="comment">-- 修改表名</span></span><br><span class="line"><span class="keyword">ALTER</span> <span class="keyword">TABLE</span> XXX <span class="keyword">ADD</span> ···<span class="comment">--增加表的字段</span></span><br><span class="line"><span class="keyword">ALTER</span> <span class="keyword">TABLE</span> XXX <span class="keyword">DROP</span> ···<span class="comment">--删除字段</span></span><br><span class="line"><span class="keyword">ALTER</span> <span class="keyword">TABLE</span> XXX MODIFY ···<span class="comment">--修改表的字段属性、约束</span></span><br><span class="line"><span class="keyword">ALTER</span> <span class="keyword">TABLE</span> XXX CHANGE ···<span class="comment">-- 表的字段重命名</span></span><br></pre></td></tr></table></figure><h2 id="2、表数据管理"><a href="#2、表数据管理" class="headerlink" title="2、表数据管理"></a>2、表数据管理</h2><pre><code> insert插入 update修改 delete删除 对象一般是`表名` 注意字段用``，写常量赋值时用' '</code></pre><h3 id="2-1、外键"><a href="#2-1、外键" class="headerlink" title="2.1、外键"></a>2.1、外键</h3><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">ALTER</span> <span class="keyword">TABLE</span> XXX</span><br><span class="line"><span class="keyword">ADD</span> <span class="keyword">CONSTRAINT</span> `FK_xxxid` <span class="keyword">FOREIGN</span> KEY(`xxxid`) <span class="keyword">REFERENCES</span> `xxx`(`xxxid`);</span><br><span class="line"><span class="comment">-- 不常用，因为外键删除时，父表和子表均要处理，联动关系很麻烦</span></span><br></pre></td></tr></table></figure><h3 id="2-2、插入"><a href="#2-2、插入" class="headerlink" title="2.2、插入"></a>2.2、插入</h3><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">insert</span> <span class="keyword">into</span> `表名`(`字段名`) <span class="keyword">values</span> (<span class="string">'值'</span>);</span><br><span class="line"><span class="comment">-- 将值赋予相应字段，位置一一对应</span></span><br></pre></td></tr></table></figure><h3 id="2-3、修改"><a href="#2-3、修改" class="headerlink" title="2.3、修改"></a>2.3、修改</h3><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">update `表名` <span class="keyword">set</span> `字段`<span class="operator">=</span>val <span class="keyword">where</span> 条件</span><br><span class="line"><span class="comment">-- 条件即指定一些判定语句，没有where来判断，则表内数据全更改</span></span><br></pre></td></tr></table></figure><h3 id="2-4、删除"><a href="#2-4、删除" class="headerlink" title="2.4、删除"></a>2.4、删除</h3><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">delete</span> <span class="keyword">from</span> `表名` <span class="keyword">where</span> 条件</span><br><span class="line"><span class="comment">-- 条件是指定删除对象，无where判断则表数据全删，自增量不变</span></span><br><span class="line"><span class="comment">-- 使用INNODB，在delete后 重启数据库，自增量从1开始，而使用MYISAM重启后自增量不变</span></span><br><span class="line"><span class="keyword">truncate</span> <span class="keyword">table</span> `表名`</span><br><span class="line"><span class="comment">-- 清表，自增量归0</span></span><br></pre></td></tr></table></figure><h2 id="3、DQL查询"><a href="#3、DQL查询" class="headerlink" title="3、DQL查询"></a>3、DQL查询</h2><h3 id="3-1、DQL"><a href="#3-1、DQL" class="headerlink" title="3.1、DQL"></a>3.1、DQL</h3><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">select</span> <span class="operator">*</span> <span class="keyword">from</span> `表名` <span class="comment">-- *代表查询全部内容</span></span><br><span class="line"><span class="keyword">select</span> `字段<span class="number">1</span>` <span class="keyword">as</span> 别名,`字段<span class="number">2</span>` <span class="keyword">as</span> 别名 <span class="keyword">from</span> `表名` <span class="keyword">as</span> 别名</span><br><span class="line"><span class="comment">-- 别名``，''带不带都行，不要别名，查询出来即设置的字段名</span></span><br><span class="line"></span><br><span class="line"><span class="comment">-- concat(a,b) 连接a，b两个字符串</span></span><br><span class="line"><span class="keyword">select</span> concat(<span class="string">'我的名字是 '</span>,`名字字段`) <span class="keyword">as</span> 别名 <span class="keyword">from</span> `表名`</span><br><span class="line"></span><br><span class="line"><span class="comment">-- distinct 去重</span></span><br><span class="line"><span class="keyword">select</span> <span class="keyword">distinct</span> `字段` <span class="keyword">from</span> `表名`</span><br><span class="line"></span><br><span class="line"><span class="keyword">SELECT</span> VERSION(); <span class="comment">-- 查询版本</span></span><br><span class="line"><span class="keyword">SELECT</span> <span class="number">455</span><span class="operator">*</span><span class="number">8484</span> <span class="keyword">AS</span> 结果; <span class="comment">-- 基本计算</span></span><br><span class="line"><span class="keyword">SELECT</span> `studentresult`<span class="operator">+</span><span class="number">1</span> <span class="keyword">AS</span> 成绩加<span class="number">1</span>分 <span class="keyword">FROM</span> `<span class="keyword">result</span>`; <span class="comment">-- 改变字段数据</span></span><br></pre></td></tr></table></figure><h3 id="3-2、where及条件语句"><a href="#3-2、where及条件语句" class="headerlink" title="3.2、where及条件语句"></a>3.2、where及条件语句</h3><h4 id="简单查询"><a href="#简单查询" class="headerlink" title="简单查询"></a>简单查询</h4><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">-- 简单逻辑运算符</span></span><br><span class="line"><span class="keyword">and</span> <span class="operator">&&</span> <span class="keyword">or</span><span class="operator">||</span> <span class="keyword">not</span>！</span><br><span class="line"></span><br><span class="line"><span class="comment">-- where 返回布尔值</span></span><br><span class="line"><span class="keyword">SELECT</span> `studentresult` <span class="keyword">AS</span> 成绩 <span class="keyword">FROM</span> `<span class="keyword">result</span>` <span class="keyword">WHERE</span> `studentresult`<span class="operator">>=</span><span class="number">80</span> <span class="keyword">AND</span> `studentresult`<span class="operator"><=</span><span class="number">90</span>;</span><br><span class="line"><span class="keyword">SELECT</span> `studentresult` <span class="keyword">AS</span> 成绩 <span class="keyword">FROM</span> `<span class="keyword">result</span>` <span class="keyword">WHERE</span> `studentresult` <span class="keyword">BETWEEN</span> <span class="number">80</span> <span class="keyword">AND</span> <span class="number">100</span>;</span><br></pre></td></tr></table></figure><h4 id="模糊查询"><a href="#模糊查询" class="headerlink" title="模糊查询"></a>模糊查询</h4><table><thead><tr><th align="center">语句</th><th align="center">描述</th></tr></thead><tbody><tr><td align="center">a not null</td><td align="center">若为空则真</td></tr><tr><td align="center">a is not null</td><td align="center">不为空则真</td></tr><tr><td align="center">a between b and c</td><td align="center">a在b与c之间为真</td></tr><tr><td align="center">a like b</td><td align="center">a匹配b成功则为真</td></tr><tr><td align="center">a in (a1,a2,a3···)</td><td align="center">括号内值有a则为真</td></tr></tbody></table><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">select</span> `字段` <span class="keyword">from</span> `表名` <span class="keyword">where</span> `字段` <span class="keyword">like</span> <span class="string">'常量'</span></span><br><span class="line"><span class="comment">-- 其中%代表0或多个任意字符，一个_为单个字符</span></span><br><span class="line"><span class="keyword">select</span> `字段` <span class="keyword">from</span> `表名` <span class="keyword">where</span> `字段` <span class="keyword">in</span> (<span class="string">'范围'</span>,<span class="string">' '</span>,<span class="string">' '</span>)</span><br><span class="line"><span class="comment">-- 找出范围内的数据</span></span><br></pre></td></tr></table></figure><h4 id="连接查询"><a href="#连接查询" class="headerlink" title="连接查询"></a>连接查询</h4><p>参考网页：<a href="https://www.cnblogs.com/mafeng/p/10150013.html">https://www.cnblogs.com/mafeng/p/10150013.html</a></p><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment">/*</span></span><br><span class="line"><span class="comment">连接不同表来查询数据;</span></span><br><span class="line"><span class="comment">连接查询 INNER/LEFT/RIGHT JOIN(连接表名) ON (判断条件);</span></span><br><span class="line"><span class="comment">这里是连接查询使用join on配套语句，条件语句不用where;</span></span><br><span class="line"><span class="comment">必须要有on;</span></span><br><span class="line"><span class="comment">多个表内有相同字段，需要标明对应的表名;</span></span><br><span class="line"><span class="comment">一般on条件为相同量。</span></span><br><span class="line"><span class="comment">*/</span> </span><br><span class="line"><span class="comment">-- 1、内连接 inner join(和join相同)</span></span><br><span class="line"><span class="keyword">SELECT</span> s.studentno,studentname <span class="keyword">FROM</span> student <span class="keyword">AS</span> s </span><br><span class="line"><span class="keyword">INNER</span> <span class="keyword">JOIN</span> <span class="keyword">result</span> <span class="keyword">AS</span> r</span><br><span class="line"><span class="keyword">ON</span> s.studentno <span class="operator">=</span> r.studentno</span><br><span class="line"><span class="comment">-- 返回符合条件语句的查询字段</span></span><br><span class="line"></span><br><span class="line"><span class="comment">-- 2、外连接 left join/right join/full join</span></span><br><span class="line"><span class="keyword">select</span> 字段<span class="number">1</span>,字段<span class="number">2</span>,字段<span class="number">3</span> <span class="keyword">from</span> 表名<span class="number">1</span></span><br><span class="line"><span class="keyword">left</span> <span class="keyword">join</span> 表名<span class="number">2</span></span><br><span class="line"><span class="keyword">on</span> 条件</span><br><span class="line"><span class="comment">-- 返回左表的全部需要字段，且返回右表符合条件语句的字段，没有的字段为null(right join:左->右),full join相当于前两个的并集。</span></span><br><span class="line"></span><br><span class="line"><span class="comment">-- 3、多表连接</span></span><br><span class="line"><span class="keyword">SELECT</span> s.studentno,s.studentname,subjectname </span><br><span class="line"><span class="keyword">FROM</span> student <span class="keyword">AS</span> s </span><br><span class="line"><span class="keyword">INNER</span> <span class="keyword">JOIN</span> <span class="keyword">result</span> <span class="keyword">AS</span> r</span><br><span class="line"><span class="keyword">ON</span> s.studentno <span class="operator">=</span> r.studentno</span><br><span class="line"></span><br><span class="line"><span class="keyword">RIGHT</span> <span class="keyword">JOIN</span> `subject` <span class="keyword">AS</span> sub</span><br><span class="line"><span class="keyword">ON</span> r.subjectno<span class="operator">=</span>sub.subjectno</span><br><span class="line"><span class="comment">-- 用join语句连接多表，每次连接均与join语句左半部分连接。</span></span><br></pre></td></tr></table></figure><h3 id="3-3、自连接"><a href="#3-3、自连接" class="headerlink" title="3.3、自连接"></a>3.3、自连接</h3><figure class="highlight sql"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"></span><br></pre></td></tr></table></figure>]]></content>
<summary type="html"><h1 id="MYSQL基础"><a href="#MYSQL基础" class="headerlink" title="MYSQL基础"></a>MYSQL基础</h1><pre><code> 数据库·····语言
DDL定义 DML操作（增删改） DQL查询 </summary>
<category term="MYSQL" scheme="http://aidianfirst.com/categories/MYSQL/"/>
<category term="学习" scheme="http://aidianfirst.com/tags/%E5%AD%A6%E4%B9%A0/"/>
</entry>
<entry>
<title>树 头指针的小细节</title>
<link href="http://aidianfirst.com/2021/04/25/%E6%A0%91%20%E5%A4%B4%E6%8C%87%E9%92%88%E7%9A%84%E5%B0%8F%E7%BB%86%E8%8A%82/"/>
<id>http://aidianfirst.com/2021/04/25/%E6%A0%91%20%E5%A4%B4%E6%8C%87%E9%92%88%E7%9A%84%E5%B0%8F%E7%BB%86%E8%8A%82/</id>
<published>2021-04-25T15:07:04.000Z</published>
<updated>2021-04-25T16:31:31.876Z</updated>
<content type="html"><![CDATA[<h1 id="树的头指针"><a href="#树的头指针" class="headerlink" title="树的头指针"></a>树的头指针</h1><p>    数据结构看完对链表头指针印象还是很深刻的，但是没有实践写过相关代码，近期leetcode刷题碰到用数据结构的中等题都歇菜了，今天是897递增顺序搜索树一个简单题，看完后有了思路，中序遍历（递归）+右子树斜树，但是写代码就犯难了，我发现我还没用java写过数据结构，之前看的大话数据结构还是用C写的，然后我便参考了宫水三叶的题解。</p><figure class="highlight java"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">TreeNode dummy = <span class="keyword">new</span> TreeNode(-<span class="number">1</span>);<span class="comment">//头指针，方便头结点找前驱结点</span></span><br><span class="line">TreeNode head = dummy;<span class="comment">//把头指针给头结点</span></span><br><span class="line"><span class="keyword">for</span> (TreeNode node : list) {</span><br><span class="line"> head.right = node;<span class="comment">//遍历排好的中序遍历list,全部给头结点的右子树</span></span><br><span class="line"> node.left = <span class="keyword">null</span>;<span class="comment">//左子树为空</span></span><br><span class="line"> head = node;<span class="comment">//把右子树设为新的头结点</span></span><br><span class="line">}</span><br><span class="line"><span class="keyword">return</span> dummy.right;<span class="comment">//头指针并不需要，只是为了头结点服务，所以直接返回其右子树即头结点树</span></span><br><span class="line"></span><br></pre></td></tr></table></figure><p>  题解里这段把我看懵了，不知道为什么要新建dummy这个节点，返回值为什么是dummy.right。在网上搜了一会儿，我找到了答案。</p><p>  dummy就是一个头指针，因为头结点没有前驱节点，dummy相当于一个工具服务头结点，用来定义头结点的。然后把dummy赋予head，开始循环赋值，最后返回值自然而然便是dummy.right，因为dummy头指针没用，从其右子树开始。</p><p>  果然还是实践出真知，以后初始化树，头指针的应用我是记下了。</p>]]></content>
<summary type="html"><h1 id="树的头指针"><a href="#树的头指针" class="headerlink" title="树的头指针"></a>树的头指针</h1><p>&ensp;&ensp;&ensp;&ensp;数据结构看完对链表头指针印象还是很深刻的，但是没有实践写过相关代码，</summary>
<category term="leetcode刷题" scheme="http://aidianfirst.com/categories/leetcode%E5%88%B7%E9%A2%98/"/>
<category term="学习" scheme="http://aidianfirst.com/tags/%E5%AD%A6%E4%B9%A0/"/>
</entry>
<entry>
<title>第一篇博客</title>
<link href="http://aidianfirst.com/2021/04/12/%E7%AC%AC%E4%B8%80%E7%AF%87%E5%8D%9A%E5%AE%A2/"/>
<id>http://aidianfirst.com/2021/04/12/%E7%AC%AC%E4%B8%80%E7%AF%87%E5%8D%9A%E5%AE%A2/</id>
<published>2021-04-12T13:07:04.000Z</published>
<updated>2021-04-25T15:49:20.218Z</updated>
<content type="html"><![CDATA[<h2 id="近况说明"><a href="#近况说明" class="headerlink" title="近况说明"></a>近况说明</h2><pre><code> 朋友你好，今后我会在博客更新自己的学习记录。 hexo博客的基础框架在2月时以搭建好，最近看完了Java编程思想才开始完善后续优化以及写博客。</code></pre><hr><h2 id="目前已简略看完的课程："><a href="#目前已简略看完的课程：" class="headerlink" title="目前已简略看完的课程："></a>目前已简略看完的课程：</h2><pre><code> 大话数据结构、计算机网络B站课程、Java编程思想。</code></pre><h2 id="后续阶段目标："><a href="#后续阶段目标：" class="headerlink" title="后续阶段目标："></a>后续阶段目标：</h2><pre><code> 剑指Offer、大话设计模式、leetcode每日一题； Mysql数据库、ssm框架、Spring boot； hexo博客完善、Java实践； 英语四级。</code></pre>]]></content>
<summary type="html"><h2 id="近况说明"><a href="#近况说明" class="headerlink" title="近况说明"></a>近况说明</h2><pre><code> 朋友你好，今后我会在博客更新自己的学习记录。
hexo博客的基础框架在2月时以搭建好，最近看完了Ja</summary>
<category term="记录生活" scheme="http://aidianfirst.com/categories/%E8%AE%B0%E5%BD%95%E7%94%9F%E6%B4%BB/"/>
<category term="日常" scheme="http://aidianfirst.com/tags/%E6%97%A5%E5%B8%B8/"/>
</entry>
</feed>
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