MySQL基础（一）——查询语句

1.导入示例数据库

2.SQL是说明？MySQL是说明？

3.查询语句 SELECT FROM

语句解释：

去重语句;

前N个语句：

CASE...END语句：

4.筛选语句 WHERE

语句解释： 运算符/通配符/操作符：

5.分组语句 GROUP BY

聚集函数： 语句解释： HAVING子句：

6.排序语句 ORDER BY

语句解释： 正序、逆序：

7.函数

时间函数： 数值函数： 字符串函数：

8.SQL注释

9.SQL代码规范

[SQL编程格式的优化建议] [https://zhuanlan.zhihu.com/p/27466166](https://zhuanlan.zhihu.com/p/27466166)      [SQL Style Guide] [https://www.sqlstyle.guide/](https://www.sqlstyle.guide/)

由于时间问题，参考了知乎专栏

1. 导入示例数据库

教程 MySQL导入示例数据库 - MySQL教程™，在上面链接中有示例数据库的下载链接，

Navicat自带导入功能，如下截图所示:

image

右键local，点击运行SQL文件，按步骤操作，成功会显示如下界面:

image

2. 查询语句 SELECT FROM

SELECT是最常用的SQL语句，没有之一，它的用途是从一个或多个表中检索信息，使用SELECT检索信息，至少要给出两条信息：

1、想选择什么信息；

2、从哪里选择信息；

实例

查询一列的数据

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n39" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT contactLastName FROM customers --contactLastName是列标签，customers是表名</pre>

运行结果如下所示，有122条，如下所示：

image

利用distinct语句去重

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n43" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT DISTINCT contactLastName FROM customers -- contactLastName是列标签，customers是表名</pre>

该查询语句在contactLastName加上distinct，只会保留唯一值，重复的会去掉，可以看到结果从122条变成108条，如下所示：

image

利用limit取前N个语句

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="text" contenteditable="true" cid="n47" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT contactLastName FROM customers LIMIT 10 -- contactLastName是列标签，customers是表名</pre>

limit后面的数字可以为任意正整数，只要小于表的行数，结果如下：

image

利用CASE...END判断语句

详细参考

mysql操作查询结果case when then else end用法举例www.cnblogs.com

4. 筛选语句 WHERE

WHERE 子句配合 SELECT 语句，表示有条件的筛选信息，类似程序语言中的if条件。

where语句常结合运算符、通配符操作，运算符常见有算术运算符、逻辑运算符、比较运算符等。

详细参考：

MySQL（六）之MySQL常用操作符 - 苦水润喉 - 博客园www.cnblogs.com!图标

通配符

image

算术运算符

“+”、“-”、“*”、“/”、“%”分别对应加减乘除求余。

比较运算符

“==”等于、“<=>”安全的等于、“<>(!=)”不等于、“<=”小于等于、“>=”大于等于、“IS NULL”判断一个值是否为NULL、“IS NOT NULL”判断一个值是否不为NULL。

逻辑运算符

NOT或者！ 逻辑非

AND或者&& 逻辑与

OR或者|| 逻辑或

XOR 逻辑异或

实例

利用where语句筛选

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n72" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT contactLastName FROM customers WHERE country = "USA" -- contactLastName是列标签，customers是表名</pre>

选取国籍是USA的contactLastName，有36条信息：

image

where语句加上AND，多个条件同时满足

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n76" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT contactLastName FROM customers WHERE country = "USA" AND salesRepEmployeeNumber > 1200-- contactLastName是列标签，customers是表名</pre>

选取国籍是USA、salesRepEmployeeNumber > 12的contactLastName，有18条信息：

image

where语句加上OR、IS NULL

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n80" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT contactLastName FROM customers WHERE country = "France" OR state IS NULL-- contactLastName是列标签，customers是表名</pre>

只要满足一个条件就行，73条数据，如下：

image

5. 分组语句 GROUP BY

GROUP BY 语句根据一个或多个列对结果集进行分组，可以理解成Excel上的透视表。HAVING子句常常配合GROUP BY使用，和where功能相似，where对行进行筛选，而HAVING则是对分组进行筛序。

MySQL中提供的聚合函数可以用来统计、求和、求最值等等。

分类：

COUNT：统计行数量 SUM：获取单个列的合计值 AVG：计算某个列的平均值 MAX：计算列的最大值 MIN：计算列的最小值

实例

按国家分类，并统计出现的次数

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n90" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT country,count(*) as num FROM customers GROUP BY country</pre>

结果如下，有27个国家：

image

利用having语句，统计出现次数大于3的国家

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n94" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT country,count() as num FROM customers GROUP BY country HAVING count() > 3</pre>

筛选之后，只有8条信息，结果如下：

image

6. 排序语句 ORDER BY

ORDER BY子句来设定你想按哪个字段哪种方式来进行排序，再返回搜索结果，常常配合ASC升序、DSEC降序使用。

实例

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n100" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT country,count() as num
FROM customers
WHERE creditLimit > 0
GROUP BY country HAVING count() > 3
ORDER BY country DESC ,count(*) ASC</pre>

GROUP BY语句常常在WHERE之后，ORDER BY之前，country是降序，国家计数是升序，结果如下：

image

\7. 函数

时间函数太多，只选部分函数

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n105" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT NOW();-- 获得当前日期
SELECT TIME('2017-05-15 10:37:14.123456');-- 获取时间
SELECT YEAR('2017-05-15 10:37:14.123456');-- 获取年份
SELECT MONTH('2017-05-15 10:37:14.123456');-- 获取月份
SELECT DAY('2017-05-15 10:37:14.123456');-- 获取日
SELECT HOUR('2017-05-15 10:37:14.123456');-- 获取时
SELECT FROM_UNIXTIME(1494815834);-- 将时间戳转为具体时间</pre>

数值函数

image

字符串函数

image

这一节实例相对简单，不做具体说明。

\8. SQL注释

“--”之后的文本属于注释。

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n113" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT NOW();-- 获得当前日期</pre>

“//”从/开始，到/结束，/和/之间的任何内容都是注释

\9. SQL代码规范

SQL编程格式的优化建议(https://zhuanlan.zhihu.com/p/27466166)

SQL Style Guide(http://link.zhihu.com/?target=https%3A//www.sqlstyle.guide/)

题目一：

数据表如下如图所示，查找重复的电子邮箱

image

利用HAVING的筛选分组功能

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n122" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT Email as NUM FROM email GROUP BY Email HAVING count(Email) > 1</pre>

输出：

image

题目二：

如果一个国家的面积超过300万平方公里，或者(人口超过2500万并且gdp超过2000万)，那么这个国家就是大国家。

编写一个SQL查询，输出表中所有大国家的名称、人口和面积。

image

利用where和运算符做筛选

<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="mysql" contenteditable="true" cid="n130" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; background-image: inherit; background-position: inherit; background-size: inherit; background-repeat: inherit; background-attachment: inherit; background-origin: inherit; background-clip: inherit; background-color: rgb(54, 59, 64); font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; position: relative !important; padding: 10px 30px; border: 1px solid; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">SELECT name,population, area FROM world WHERE area > 3000000 OR (population > 25000000 AND gdp > 20000000)</pre>

结果如下，和预期一致：