1、select
1.1 作用
获取mysql中的数据行
1.2 单独使用select
1.2.1 select @@XXX；获取参数信息
select @@port; 查端口号
show variables; 查看所有参数
show variables like '%innodb%'; |查参数
1.2.2 select 函数（）；
select now(); 函数加括号
mysql> select database(); |库
mysql> select now(); |时间
mysql> select version(); |版本
1.3 sql92标准的使用语法
1.3.1 select语法执行顺序
select开始--> from 字句 --> where子句 -->
group by 子句 --> select 后执行条件-->
having子句 --> order by -->limit| desc city | 表结构 |
|---|---|
| id | 自增的无关列 |
| name | 城市名字 |
| countrycode | 所在国家代号 |
| district | 中国省的意思 每个是洲的意思 |
| populiation | 城市人口数量 |
| select * from jyt; | 相对路径查询 生产中使用较少 |
|---|---|
| select * from root.jyt | 绝对路径 生产中使用较少 |
| select name,populication from jyt; | 查看两列的内容 |
| select name,populication from root.jyt; | 查看两列的内容 |
| where | 相当于grep | 说明 |
|---|---|---|
| where配合等值查询 | select * from world.city where countrycode=‘chn‘; | 查询表中的中国城市信息 |
| where配合不等值查询 | select * from world.city where Population<100; | 人口小于100人的城市 （>,<,<=,>=,<>） |
| where配合模糊查询 | select * from world.city where CountryCode like ‘c%‘; | 国家以c开头 禁止%开头 |
| where配合逻辑连接符（AND or） | select * from world.city where Population > 10000 AND Population < 20000; | select * from world.city where population between 10000 and 20000; |
| select * from world.city where CountryCode=‘chn‘ OR CountryCode=‘usa‘; | select * from world.city where countrycode in (‘chn‘,‘usa‘); | |
| SELECT * FROM world.city WHERE CountryCode=‘chn‘ UNION ALL SELECT*FROM world.city WHERE CountryCode=‘usa‘; | 推荐 union 去重 加all不去重 默认去重 |
常用聚合函数
| 函数 | 例子 |
|---|---|
| avg() | select district,avg(population) from city where countrycode=‘chn‘ group by district; |
| count() | select countrycode,count(name) from city group by countrycode; |
| sum() | select countrycode,sum(population) from city group by countrycode ; |
| max() | - |
| min() | - |
| group_concat() | select countrycode,group_concat(district) from city group by countrycode; |
| order by | 排序 |
|---|---|
| 查询统计总数 | select district,sum(population) from city where countrycode=‘chn‘ group by district; |
| 查询统计总数并排序降序 | SELECT district,sum(population) FROM city WHERE countrycode=‘chn‘ GROUP BY district ORDER BY SUM(Population) DESC; |
| 查询中国所有的城市，并以人口数降序输出 | select*from city where countrycode=‘chn‘ order by population desc; |
| - | - |
| limit m,n 跳过m行显示n行 | limit x offset y 跳过y行显示x行 |
| 前5行 | SELECT*FROM city WHERE countrycode=‘chn‘ ORDER BY population DESC LIMIT 5; |
| 显示6-10行 | SELECT*FROM city WHERE countrycode=‘chn‘ ORDER BY population DESC LIMIT 5,5; |
| 显示6-10行 | select*from city where countrycode=‘chn‘ order by population desc limit 5 offset 5; |
原文地址：https://www.cnblogs.com/jiangyatao/p/11044684.html
时间： 2024-08-29 19:49:54