select应用场景:
1单独使用;
查看端口号:select @@port;
查看用户和主机:select user,host from mysql.user;
格式化查看用户和主机:select user,host from mysql.user\G;
模糊查询含有innodb的命令:show variables like '%innodb%'
2配合函数使用:
查时间:select now();
查询当前所在的库: select database();
查询库名:
mysql> show databases;
ERROR 2006 (HY000): MySQL server has gone away
No connection. Trying to reconnect...
Connection id: 47
Current database: *** NONE ***
+--------------------+
| Database |
+--------------------+
| information_schema |
| mysql |
| performance_schema |
| sys |
| tangTest |
| testku |
+--------------------+
6 rows in set (0.04 sec)
使用mysql这个库:
mysql> use mysql
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A
Database changed
查看当前的库名:
mysql> select database();
+------------+
| database() |
+------------+
| mysql |
+------------+
1 row in set (0.00 sec)
查看当前数据库版本:
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.26-log |
+------------+
1 row in set (0.00 sec)
mysql>
SQL92标准的使用语法
select语法执行顺序:
select开始--->from子句--->where子句--->group by子句--->select后执行条件--->having子句--->order by --->limit
--- select语句应用
1进入world这个库,查询city表的所以数据:
相对路径写法:
use world;
select * from city;
绝对路径写法:
select * from world.city;
2查询name和populaton这两个列的所有值:
相对路径写法:
use world
select name, population from city;
绝对路径写法:
select name, population from world.city;
3查询库里所有的表:
show tables from world;
4列出表中的信息:
DSC city;
---where语句用法:
1等值查询(=)
查询中国城市信息
select * from world.city where countrycode='CHN';
2不等值查询(>,<,>=,<=,<>)
查询世界人口小于100人的城市
select * from world.city where population<100;
3模糊查询(需配合like)
查询国家代号是c开头的城市:
select * from world.city where countrycode like 'c%';
统计国家代码以c开头的城市一共有多少行?
select count(*) from world.city where countrycode like 'c%';
4配合逻辑连接符查询(and ,or)
统计大于等于1万小于等于3万的城市
select * from world.city where population >= 10000 and population <= 20000;
select * from world.city where population between 10000 and 20000;
5查询一下中国或美国的城市信息
select * from world.city where countrycode='CHN' or countrycode='USA';
select * from world.city where countrycode in ('CHN','USA');
6查询一下中国或美国的城市信息(推荐):
union和union all的意思是聚合;但union可以去重,union all不去重。
select * from world where countrycode='CHN' union all select * from world.city where countrycode='USA'
-- group by 配合聚合函数应用
常用的聚合函数:
avg()
count()
max()
min()
group_concat()
1统计世界上每个国家的人口数:
解释:以国家代码进行分组,以国家代码进行聚合运算
select countrycode,sum(population) from world.city group by countrycode;
2统计每个国家的城市个数
字段name是城市名的字段
select countrycode, count(name) from world.city group by countrycode;
3统计每个国家的省名列表
count()函数表示将多个字符串连接成一个字符串,而group_concat()则是以group by为条件拼接字符串
这里的district是省的字段名
select coutrycode,group_concat(district) from world.city group_by countrycode;
4统计中国每个省的城市名列表
select district, group_concat(name) from worls.city where countrycode='CHN' group by district;
5统计中国每个省的总人口数
select discrit, sum(population) from world.city where countrycode='CHN' gropy by district;
6统计一下中国每个省的平均人口数
select dictrict, avg(population) from world.city where countrycode='CHN' group by district;
-- having:对处理的结果进行再过滤(后过滤):
统计中国的每个省的人口数量大于1000万的省人口数
select district,sum(population) from world.city where countrycode='CHN' group by district having sum(population)>10000000;
统计中国每个省的总人口数并从小到大排序
select district sum(population) from world.city where countrycode='CHN' group by district order by sum(population);
从大到小排列中国每个省的人口数
select district sum(population) from world.city where countrycode='CHN' group by district order by dum(population)
DESC;
查询中国所有城市,并以人口数降序排列
select * from world.city where countrycode='CHN' order by popelation DESC;
-- limit:分页显示(一般都配合order by一起使用):
查询中国所有城市,以人口数量升序排列,只显示前五
select * from world.city where countrycode='CHN' order by population limit 5;
查询中国所有城市,以人口数量升序排列,显示6-8行
limit 5,5:第一个5表示跳过前5行,第二个3表示显示3行,其实就是显示6-8行。
select * from world.city where countrycode='CHN' order by population limit 5,3;
查询中国所有城市,以人口数量升序排列,显示6-10行
limit 5 offset 5:第一个5表示显示5行,第二个5表示偏移5行
select * from world.city where countrycode='CHN' order by population limit 5 offset 5;