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对于分析人员来讲，Mysql数据库应用最多的是select查询语句，此篇文章主要介绍Mysql数据库的查询语句。
一、单表查询
1.带条件的查询
基本语法：select * from +表名称 +where 条件;
1)范围查询：
eg：where 字段 between 10 and 100;
2)模糊查询
eg：where 字段 like'%不确定%'
备注：%代表多个字符，_下划线代表一个字符，^a代表以a为开头的数据，a$代表以a为结尾的数据，[abc]匹配所包含的任意一个字符。
2.多条件查询
备注：当and和or同时使用时，and优先级高于or。如果想要优先带or的条件，对带or部分条件带括号即可。
3.排序
基本语法：order by 字段。
备注：默认是升序，ASC升序，DESC降序
限制记录数：limit 100
4.聚合函数
count()：统计记录数
avg():平均数
max()：最大值
min()：最小值
sum()：求和
5.分组
基本语法：select 字段1，字段2，聚合函数 from +表名称 +group by 字段1，字段2
备注：group by和having 一起使用，主要是对分组结果进行过滤
二、多表关联查询
1.内连接-->inner join
内连接即等值连接，获取两个表中字段匹配关系的记录，可省略写成join，
可理解成集合概念中的“交集”，关联字段同时存在与两表的记录。
2.左连接-->left join
左连接，获取左边主表的全部记录，即便右表没有对应的数据。
3.右连接-->right join
右连接，获取右边主表的全部记录，即便左表没有对应的数据。
4.连接符-->union
union用于连接两个以上的select语句的结果，将结果组合到一个结果集中，并删除重复数据
基本语法：select 字段 from 表1 union [all|distinct] select 字段 from 表2
备注：union 即为 union distinct；若为union
all,即返回带重复数据的结果集；在使用union时，所选出的内容显示会以前面的表的字段名称命名。
