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VALUES (值1, 值2, ...);</code> 全部字段： <code>INSERT INTO 表名 VALUES (值1, 值2, ...);</code></p><p>批量添加数据： <code>INSERT INTO 表名 (字段名1, 字段名2, ...) VALUES (值1, 值2, ...), (值1, 值2, ...), (值1, 值2, ...);</code><code>INSERT INTO 表名 VALUES (值1, 值2, ...), (值1, 值2, ...), (值1, 值2, ...);</code></p><h5 id="注意事项-1" tabindex="-1"><a class="header-anchor" href="#注意事项-1"><span>注意事项</span></a></h5><ul><li>字符串和日期类型数据应该包含在引号中</li><li>插入的数据大小应该在字段的规定范围内</li></ul><h4 id="更新和删除数据" tabindex="-1"><a class="header-anchor" href="#更新和删除数据"><span>更新和删除数据</span></a></h4><p>修改数据： <code>UPDATE 表名 SET 字段名1 = 值1, 字段名2 = 值2, ... [ WHERE 条件 ];</code> 例： <code>UPDATE emp SET name = 'Jack' WHERE id = 1;</code></p><p>删除数据： <code>DELETE FROM 表名 [ WHERE 条件 ];</code></p><h3 id="dql-数据查询语言" tabindex="-1"><a class="header-anchor" href="#dql-数据查询语言"><span>DQL（数据查询语言）</span></a></h3><p>语法：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">SELECT</span> <span class="line"> 字段列表</span> <span class="line">FROM</span> <span class="line"> 表名字段</span> <span class="line">WHERE</span> <span class="line"> 条件列表</span> <span class="line">GROUP BY</span> <span class="line"> 分组字段列表</span> <span class="line">HAVING</span> <span class="line"> 分组后的条件列表</span> <span class="line">ORDER BY</span> <span class="line"> 排序字段列表</span> <span class="line">LIMIT</span> <span class="line"> 分页参数</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h4 id="基础查询" tabindex="-1"><a class="header-anchor" href="#基础查询"><span>基础查询</span></a></h4><p>查询多个字段： <code>SELECT 字段1, 字段2, 字段3, ... FROM 表名;</code><code>SELECT * FROM 表名;</code></p><p>设置别名： <code>SELECT 字段1 [ AS 别名1 ], 字段2 [ AS 别名2 ], 字段3 [ AS 别名3 ], ... FROM 表名;</code><code>SELECT 字段1 [ 别名1 ], 字段2 [ 别名2 ], 字段3 [ 别名3 ], ... FROM 表名;</code></p><p>去除重复记录： <code>SELECT DISTINCT 字段列表 FROM 表名;</code></p><p>转义： <code>SELECT * FROM 表名 WHERE name LIKE '/_张三' ESCAPE '/'</code> / 之后的_不作为通配符</p><h4 id="条件查询" tabindex="-1"><a class="header-anchor" href="#条件查询"><span>条件查询</span></a></h4><p>语法： <code>SELECT 字段列表 FROM 表名 WHERE 条件列表;</code></p><p>条件：</p><table><thead><tr><th>比较运算符</th><th>功能</th></tr></thead><tbody><tr><td>></td><td>大于</td></tr><tr><td>>=</td><td>大于等于</td></tr><tr><td><</td><td>小于</td></tr><tr><td><=</td><td>小于等于</td></tr><tr><td>=</td><td>等于</td></tr><tr><td><> 或 !=</td><td>不等于</td></tr><tr><td>BETWEEN ... AND ...</td><td>在某个范围内（含最小、最大值）</td></tr><tr><td>IN(...)</td><td>在in之后的列表中的值，多选一</td></tr><tr><td>LIKE 占位符</td><td>模糊匹配（_匹配单个字符，%匹配任意个字符）</td></tr><tr><td>IS NULL</td><td>是NULL</td></tr></tbody></table><table><thead><tr><th>逻辑运算符</th><th>功能</th></tr></thead><tbody><tr><td>AND 或 &&</td><td>并且（多个条件同时成立）</td></tr><tr><td>OR 或 ||</td><td>或者（多个条件任意一个成立）</td></tr><tr><td>NOT 或 !</td><td>非，不是</td></tr></tbody></table><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 年龄等于30</span> <span class="line">select * from employee where age = 30;</span> <span class="line">-- 年龄小于30</span> <span class="line">select * from employee where age < 30;</span> <span class="line">-- 小于等于</span> <span class="line">select * from employee where age <= 30;</span> <span class="line">-- 没有身份证</span> <span class="line">select * from employee where idcard is null or idcard = '';</span> <span class="line">-- 有身份证</span> <span class="line">select * from employee where idcard;</span> <span class="line">select * from employee where idcard is not null;</span> <span class="line">-- 不等于</span> <span class="line">select * from employee where age != 30;</span> <span class="line">-- 年龄在20到30之间</span> <span class="line">select * from employee where age between 20 and 30;</span> <span class="line">select * from employee where age >= 20 and age <= 30;</span> <span class="line">-- 下面语句不报错，但查不到任何信息</span> <span class="line">select * from employee where age between 30 and 20;</span> <span class="line">-- 性别为女且年龄小于30</span> <span class="line">select * from employee where age < 30 and gender = '女';</span> <span class="line">-- 年龄等于25或30或35</span> <span class="line">select * from employee where age = 25 or age = 30 or age = 35;</span> <span class="line">select * from employee where age in (25, 30, 35);</span> <span class="line">-- 姓名为两个字</span> <span class="line">select * from employee where name like '__';</span> <span class="line">-- 身份证最后为X</span> <span class="line">select * from employee where idcard like '%X';</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h4 id="聚合查询-聚合函数" tabindex="-1"><a class="header-anchor" href="#聚合查询-聚合函数"><span>聚合查询（聚合函数）</span></a></h4><p>常见聚合函数：</p><table><thead><tr><th>函数</th><th>功能</th></tr></thead><tbody><tr><td>count</td><td>统计数量</td></tr><tr><td>max</td><td>最大值</td></tr><tr><td>min</td><td>最小值</td></tr><tr><td>avg</td><td>平均值</td></tr><tr><td>sum</td><td>求和</td></tr></tbody></table><p>语法： <code>SELECT 聚合函数(字段列表) FROM 表名;</code> 例： <code>SELECT count(id) from employee where workaddress = "广东省";</code></p><h4 id="分组查询" tabindex="-1"><a class="header-anchor" href="#分组查询"><span>分组查询</span></a></h4><p>语法： <code>SELECT 字段列表 FROM 表名 [ WHERE 条件 ] GROUP BY 分组字段名 [ HAVING 分组后的过滤条件 ];</code></p><p>where 和 having 的区别：</p><ul><li>执行时机不同：where是分组之前进行过滤，不满足where条件不参与分组；having是分组后对结果进行过滤。</li><li>判断条件不同：where不能对聚合函数进行判断，而having可以。</li></ul><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 根据性别分组，统计男性和女性数量（只显示分组数量，不显示哪个是男哪个是女）</span> <span class="line">select count(*) from employee group by gender;</span> <span class="line">-- 根据性别分组，统计男性和女性数量</span> <span class="line">select gender, count(*) from employee group by gender;</span> <span class="line">-- 根据性别分组，统计男性和女性的平均年龄</span> <span class="line">select gender, avg(age) from employee group by gender;</span> <span class="line">-- 年龄小于45，并根据工作地址分组</span> <span class="line">select workaddress, count(*) from employee where age < 45 group by workaddress;</span> <span class="line">-- 年龄小于45，并根据工作地址分组，获取员工数量大于等于3的工作地址</span> <span class="line">select workaddress, count(*) address_count from employee where age < 45 group by workaddress having address_count >= 3;</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h5 id="注意事项-2" tabindex="-1"><a class="header-anchor" href="#注意事项-2"><span>注意事项</span></a></h5><ul><li>执行顺序：where > 聚合函数 > having</li><li>分组之后，查询的字段一般为聚合函数和分组字段，查询其他字段无任何意义</li></ul><h4 id="排序查询" tabindex="-1"><a class="header-anchor" href="#排序查询"><span>排序查询</span></a></h4><p>语法： <code>SELECT 字段列表 FROM 表名 ORDER BY 字段1 排序方式1, 字段2 排序方式2;</code></p><p>排序方式：</p><ul><li>ASC: 升序（默认）</li><li>DESC: 降序</li></ul><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 根据年龄升序排序</span> <span class="line">SELECT * FROM employee ORDER BY age ASC;</span> <span class="line">SELECT * FROM employee ORDER BY age;</span> <span class="line">-- 两字段排序，根据年龄升序排序，入职时间降序排序</span> <span class="line">SELECT * FROM employee ORDER BY age ASC, entrydate DESC;</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h5 id="注意事项-3" tabindex="-1"><a class="header-anchor" href="#注意事项-3"><span>注意事项</span></a></h5><p>如果是多字段排序，当第一个字段值相同时，才会根据第二个字段进行排序</p><h4 id="分页查询" tabindex="-1"><a class="header-anchor" href="#分页查询"><span>分页查询</span></a></h4><p>语法： <code>SELECT 字段列表 FROM 表名 LIMIT 起始索引, 查询记录数;</code></p><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 查询第一页数据，展示10条</span> <span class="line">SELECT * FROM employee LIMIT 0, 10;</span> <span class="line">-- 查询第二页</span> <span class="line">SELECT * FROM employee LIMIT 10, 10;</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h5 id="注意事项-4" tabindex="-1"><a class="header-anchor" href="#注意事项-4"><span>注意事项</span></a></h5><ul><li>起始索引从0开始，起始索引 = （查询页码 - 1） * 每页显示记录数</li><li>分页查询是数据库的方言，不同数据库有不同实现，MySQL是LIMIT</li><li>如果查询的是第一页数据，起始索引可以省略，直接简写 LIMIT 10</li></ul><h4 id="dql执行顺序" tabindex="-1"><a class="header-anchor" href="#dql执行顺序"><span>DQL执行顺序</span></a></h4><p>FROM -> WHERE -> GROUP BY -> SELECT -> ORDER BY -> LIMIT</p><h3 id="dcl" tabindex="-1"><a class="header-anchor" href="#dcl"><span>DCL</span></a></h3><h4 id="管理用户" tabindex="-1"><a class="header-anchor" href="#管理用户"><span>管理用户</span></a></h4><p>查询用户：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">USE mysql;</span> <span class="line">SELECT * FROM user;</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div></div></div><p>创建用户: <code>CREATE USER '用户名'@'主机名' IDENTIFIED BY '密码';</code></p><p>修改用户密码： <code>ALTER USER '用户名'@'主机名' IDENTIFIED WITH mysql_native_password BY '新密码';</code></p><p>删除用户： <code>DROP USER '用户名'@'主机名';</code></p><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 创建用户test，只能在当前主机localhost访问</span> <span class="line">create user 'test'@'localhost' identified by '123456';</span> <span class="line">-- 创建用户test，能在任意主机访问</span> <span class="line">create user 'test'@'%' identified by '123456';</span> <span class="line">create user 'test' identified by '123456';</span> <span class="line">-- 修改密码</span> <span class="line">alter user 'test'@'localhost' identified with mysql_native_password by '1234';</span> <span class="line">-- 删除用户</span> <span class="line">drop user 'test'@'localhost';</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h5 id="注意事项-5" tabindex="-1"><a class="header-anchor" href="#注意事项-5"><span>注意事项</span></a></h5><ul><li>主机名可以使用 % 通配</li></ul><h4 id="权限控制" tabindex="-1"><a class="header-anchor" href="#权限控制"><span>权限控制</span></a></h4><p>常用权限：</p><table><thead><tr><th>权限</th><th>说明</th></tr></thead><tbody><tr><td>ALL, ALL PRIVILEGES</td><td>所有权限</td></tr><tr><td>SELECT</td><td>查询数据</td></tr><tr><td>INSERT</td><td>插入数据</td></tr><tr><td>UPDATE</td><td>修改数据</td></tr><tr><td>DELETE</td><td>删除数据</td></tr><tr><td>ALTER</td><td>修改表</td></tr><tr><td>DROP</td><td>删除数据库/表/视图</td></tr><tr><td>CREATE</td><td>创建数据库/表</td></tr></tbody></table><p>更多权限请看<a href="#%E6%9D%83%E9%99%90%E4%B8%80%E8%A7%88%E8%A1%A8" title="权限一览表">权限一览表</a></p><p>查询权限： <code>SHOW GRANTS FOR '用户名'@'主机名';</code></p><p>授予权限： <code>GRANT 权限列表 ON 数据库名.表名 TO '用户名'@'主机名';</code></p><p>撤销权限： <code>REVOKE 权限列表 ON 数据库名.表名 FROM '用户名'@'主机名';</code></p><h5 id="注意事项-6" tabindex="-1"><a class="header-anchor" href="#注意事项-6"><span>注意事项</span></a></h5><ul><li>多个权限用逗号分隔</li><li>授权时，数据库名和表名可以用 * 进行通配，代表所有</li></ul><h2 id="函数" tabindex="-1"><a class="header-anchor" href="#函数"><span>函数</span></a></h2><ul><li>字符串函数</li><li>数值函数</li><li>日期函数</li><li>流程函数</li></ul><h3 id="字符串函数" tabindex="-1"><a class="header-anchor" href="#字符串函数"><span>字符串函数</span></a></h3><p>常用函数：</p><table><thead><tr><th>函数</th><th>功能</th></tr></thead><tbody><tr><td>CONCAT(s1, s2, ..., sn)</td><td>字符串拼接，将s1, s2, ..., sn拼接成一个字符串</td></tr><tr><td>LOWER(str)</td><td>将字符串全部转为小写</td></tr><tr><td>UPPER(str)</td><td>将字符串全部转为大写</td></tr><tr><td>LPAD(str, n, pad)</td><td>左填充，用字符串pad对str的左边进行填充，达到n个字符串长度</td></tr><tr><td>RPAD(str, n, pad)</td><td>右填充，用字符串pad对str的右边进行填充，达到n个字符串长度</td></tr><tr><td>TRIM(str)</td><td>去掉字符串头部和尾部的空格</td></tr><tr><td>SUBSTRING(str, start, len)</td><td>返回从字符串str从start位置起的len个长度的字符串</td></tr><tr><td>REPLACE(column, source, replace)</td><td>替换字符串</td></tr></tbody></table><p>使用示例：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 拼接</span> <span class="line">SELECT CONCAT('Hello', 'World');</span> <span class="line">-- 小写</span> <span class="line">SELECT LOWER('Hello');</span> <span class="line">-- 大写</span> <span class="line">SELECT UPPER('Hello');</span> <span class="line">-- 左填充</span> <span class="line">SELECT LPAD('01', 5, '-');</span> <span class="line">-- 右填充</span> <span class="line">SELECT RPAD('01', 5, '-');</span> <span class="line">-- 去除空格</span> <span class="line">SELECT TRIM(' Hello World ');</span> <span class="line">-- 切片（起始索引为1）</span> <span class="line">SELECT SUBSTRING('Hello World', 1, 5);</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="数值函数" tabindex="-1"><a class="header-anchor" href="#数值函数"><span>数值函数</span></a></h3><p>常见函数：</p><table><thead><tr><th>函数</th><th>功能</th></tr></thead><tbody><tr><td>CEIL(x)</td><td>向上取整</td></tr><tr><td>FLOOR(x)</td><td>向下取整</td></tr><tr><td>MOD(x, y)</td><td>返回x/y的模</td></tr><tr><td>RAND()</td><td>返回0~1内的随机数</td></tr><tr><td>ROUND(x, y)</td><td>求参数x的四舍五入值，保留y位小数</td></tr></tbody></table><h3 id="日期函数" tabindex="-1"><a class="header-anchor" href="#日期函数"><span>日期函数</span></a></h3><p>常用函数：</p><table><thead><tr><th>函数</th><th>功能</th></tr></thead><tbody><tr><td>CURDATE()</td><td>返回当前日期</td></tr><tr><td>CURTIME()</td><td>返回当前时间</td></tr><tr><td>NOW()</td><td>返回当前日期和时间</td></tr><tr><td>YEAR(date)</td><td>获取指定date的年份</td></tr><tr><td>MONTH(date)</td><td>获取指定date的月份</td></tr><tr><td>DAY(date)</td><td>获取指定date的日期</td></tr><tr><td>DATE_ADD(date, INTERVAL expr type)</td><td>返回一个日期/时间值加上一个时间间隔expr后的时间值</td></tr><tr><td>DATEDIFF(date1, date2)</td><td>返回起始时间date1和结束时间date2之间的天数</td></tr></tbody></table><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- DATE_ADD</span> <span class="line">SELECT DATE_ADD(NOW(), INTERVAL 70 YEAR);</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="流程函数" tabindex="-1"><a class="header-anchor" href="#流程函数"><span>流程函数</span></a></h3><p>常用函数：</p><table><thead><tr><th>函数</th><th>功能</th></tr></thead><tbody><tr><td>IF(value, t, f)</td><td>如果value为true，则返回t，否则返回f</td></tr><tr><td>IFNULL(value1, value2)</td><td>如果value1不为空，返回value1，否则返回value2</td></tr><tr><td>CASE WHEN [ val1 ] THEN [ res1 ] ... ELSE [ default ] END</td><td>如果val1为true，返回res1，... 否则返回default默认值</td></tr><tr><td>CASE [ expr ] WHEN [ val1 ] THEN [ res1 ] ... ELSE [ default ] END</td><td>如果expr的值等于val1，返回res1，... 否则返回default默认值</td></tr></tbody></table><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">select</span> <span class="line"> name,</span> <span class="line"> (case when age > 30 then '中年' else '青年' end)</span> <span class="line">from employee;</span> <span class="line">select</span> <span class="line"> name,</span> <span class="line"> (case workaddress when '北京市' then '一线城市' when '上海市' then '一线城市' else '二线城市' end) as '工作地址'</span> <span class="line">from employee;</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h2 id="约束" tabindex="-1"><a class="header-anchor" href="#约束"><span>约束</span></a></h2><p>分类：</p><table><thead><tr><th>约束</th><th>描述</th><th>关键字</th></tr></thead><tbody><tr><td>非空约束</td><td>限制该字段的数据不能为null</td><td>NOT NULL</td></tr><tr><td>唯一约束</td><td>保证该字段的所有数据都是唯一、不重复的</td><td>UNIQUE</td></tr><tr><td>主键约束</td><td>主键是一行数据的唯一标识，要求非空且唯一</td><td>PRIMARY KEY</td></tr><tr><td>默认约束</td><td>保存数据时，如果未指定该字段的值，则采用默认值</td><td>DEFAULT</td></tr><tr><td>检查约束（8.0.1版本后）</td><td>保证字段值满足某一个条件</td><td>CHECK</td></tr><tr><td>外键约束</td><td>用来让两张图的数据之间建立连接，保证数据的一致性和完整性</td><td>FOREIGN KEY</td></tr></tbody></table><p>约束是作用于表中字段上的，可以再创建表/修改表的时候添加约束。</p><h3 id="常用约束" tabindex="-1"><a class="header-anchor" href="#常用约束"><span>常用约束</span></a></h3><table><thead><tr><th>约束条件</th><th>关键字</th></tr></thead><tbody><tr><td>主键</td><td>PRIMARY KEY</td></tr><tr><td>自动增长</td><td>AUTO_INCREMENT</td></tr><tr><td>不为空</td><td>NOT NULL</td></tr><tr><td>唯一</td><td>UNIQUE</td></tr><tr><td>逻辑条件</td><td>CHECK</td></tr><tr><td>默认值</td><td>DEFAULT</td></tr></tbody></table><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">create table user(</span> <span class="line"> id int primary key auto_increment,</span> <span class="line"> name varchar(10) not null unique,</span> <span class="line"> age int check(age > 0 and age < 120),</span> <span class="line"> status char(1) default '1',</span> <span class="line"> gender char(1)</span> <span class="line">);</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><h3 id="外键约束" tabindex="-1"><a class="header-anchor" href="#外键约束"><span>外键约束</span></a></h3><p>添加外键：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">CREATE TABLE 表名(</span> <span class="line"> 字段名字段类型,</span> <span class="line"> ...</span> <span class="line"> [CONSTRAINT] [外键名称] FOREIGN KEY(外键字段名) REFERENCES 主表(主表列名)</span> <span class="line">);</span> <span class="line">ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段名) REFERENCES 主表(主表列名);</span> <span class="line"></span> <span class="line">-- 例子</span> <span class="line">alter table emp add constraint fk_emp_dept_id foreign key(dept_id) references dept(id);</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div><div class="line-number"></div></div></div><p>删除外键： <code>ALTER TABLE 表名 DROP FOREIGN KEY 外键名;</code></p><h4 id="删除-更新行为" tabindex="-1"><a class="header-anchor" href="#删除-更新行为"><span>删除/更新行为</span></a></h4><table><thead><tr><th>行为</th><th>说明</th></tr></thead><tbody><tr><td>NO ACTION</td><td>当在父表中删除/更新对应记录时，首先检查该记录是否有对应外键，如果有则不允许删除/更新（与RESTRICT一致）</td></tr><tr><td>RESTRICT</td><td>当在父表中删除/更新对应记录时，首先检查该记录是否有对应外键，如果有则不允许删除/更新（与NO ACTION一致）</td></tr><tr><td>CASCADE</td><td>当在父表中删除/更新对应记录时，首先检查该记录是否有对应外键，如果有则也删除/更新外键在子表中的记录</td></tr><tr><td>SET NULL</td><td>当在父表中删除/更新对应记录时，首先检查该记录是否有对应外键，如果有则设置子表中该外键值为null（要求该外键允许为null）</td></tr><tr><td>SET DEFAULT</td><td>父表有变更时，子表将外键设为一个默认值（Innodb不支持）</td></tr></tbody></table><p>更改删除/更新行为： <code>ALTER TABLE 表名 ADD CONSTRAINT 外键名称 FOREIGN KEY (外键字段) REFERENCES 主表名(主表字段名) ON UPDATE 行为 ON DELETE 行为;</code></p><h2 id="多表查询" tabindex="-1"><a class="header-anchor" href="#多表查询"><span>多表查询</span></a></h2><h3 id="多表关系" tabindex="-1"><a class="header-anchor" href="#多表关系"><span>多表关系</span></a></h3><ul><li>一对多（多对一）</li><li>多对多</li><li>一对一</li></ul><h4 id="一对多" tabindex="-1"><a class="header-anchor" href="#一对多"><span>一对多</span></a></h4><p>案例：部门与员工关系：一个部门对应多个员工，一个员工对应一个部门实现：在多的一方建立外键，指向一的一方的主键</p><h4 id="多对多" tabindex="-1"><a class="header-anchor" href="#多对多"><span>多对多</span></a></h4><p>案例：学生与课程关系：一个学生可以选多门课程，一门课程也可以供多个学生选修实现：建立第三张中间表，中间表至少包含两个外键，分别关联两方主键</p><h4 id="一对一" tabindex="-1"><a class="header-anchor" href="#一对一"><span>一对一</span></a></h4><p>案例：用户与用户详情关系：一对一关系，多用于单表拆分，将一张表的基础字段放在一张表中，其他详情字段放在另一张表中，以提升操作效率实现：在任意一方加入外键，关联另外一方的主键，并且设置外键为唯一的（UNIQUE）</p><h3 id="查询" tabindex="-1"><a class="header-anchor" href="#查询"><span>查询</span></a></h3><p>合并查询（笛卡尔积，会展示所有组合结果）： <code>select * from employee, dept;</code></p><blockquote><p>笛卡尔积：两个集合A集合和B集合的所有组合情况（在多表查询时，需要消除无效的笛卡尔积）</p></blockquote><p>消除无效笛卡尔积： <code>select * from employee, dept where employee.dept = dept.id;</code></p><h3 id="内连接查询" tabindex="-1"><a class="header-anchor" href="#内连接查询"><span>内连接查询</span></a></h3><p>内连接查询的是两张表交集的部分</p><p>隐式内连接： <code>SELECT 字段列表 FROM 表1, 表2 WHERE 条件 ...;</code></p><p>显式内连接： <code>SELECT 字段列表 FROM 表1 [ INNER ] JOIN 表2 ON 连接条件 ...;</code></p><p>显式性能比隐式高</p><p>例子：</p><div class="language-mysql line-numbers-mode" data-highlighter="prismjs" data-ext="mysql" data-title="mysql"><pre class="language-mysql"><code><span class="line">-- 查询员工姓名，及关联的部门的名称</span> <span class="line">-- 隐式</span> <span class="line">select e.name, d.name from employee as e, dept as d where e.dept = d.id;</span> <span class="line">-- 显式</span> <span class="line">select e.name, d.name from employee as e inner join dept as d on e.dept = d.id;</span> <span class="line"></span></code></pre><div class="line-numbers" aria-hidden="true" style="counter-reset:line-number 0;"><div class="line-number"></div><div class="line-number"></div><div class="lin