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:
image2. 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:
imagewhere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:
imagewhere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:
image5. 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:
image6. 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:
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