day 04 DQL数据库查询语言
上集回顾:
- create table like 只能复制表结构,那有什么命令可以连数据一起复制?
(1) 方法一:
create table stu select * from student;
主键等特性没有被复制.
(2) 方法二:
create table st like student;
insert into st select * from student
pt 索引章节之后
pt-archivher
pt-osc主键是干什么用的
(1) 约束 : unique not null
(2) 聚簇索引索引: 组织和存储数据.
(3) 加速查询update 和 delete 范围操作
update t1 set xname='王二麻子' where xname like '王二%'
delete from t1 where xname like '王二%';
delete from t1 where xid >1 and xid <5 ;
delete from t1 where xid between 1 and 5 ;
===========================================
DQL 数据查询语言. ---oldguo
- SELECT
1.1 SELECT 单独使用 ***
(1) 查询数据库的参数.
SELECT @@port;
SELECT @@datadir;
SELECT @@basedir;
SELECT @@innodb_flush_log_at_trx_commit;
SHOW VARIABLES LIKE '%trx%';
(2) 调用内置函数.
USE oldguo
SELECT DATABASE();
SELECT NOW();
SELECT CONCAT(USER,"@",HOST) FROM mysql.user;
SELECT GROUP_CONCAT(xid) FROM student;
SELECT SUM(xid) FROM student;
(3) 简易计算器
SELECT 4*5;
-- 1.2 select 配合其他子句使用 *****
-- 1.2.1 子句列表介绍
FROM -- 查询对象(表,视图)
WHERE -- 过滤子句(grep)
GROUP BY -- 分组子句(统计分析类)
HAVING -- 后过滤子句
ORDER BY -- 排序子句
LIMIT -- 限制子句(分页子句)
-- 1.2.2 配合FROM应用
-- world 模板库介绍
--- 英文单词介绍
--- city -- 城市
--- id -- 序号ID主键
--- NAME -- 城市名
--- Countrycode -- 国家代码(CHN,USA,JPN)
--- District -- 省,州
--- Population -- 城市人口数
-- 例子:
-- 1.查询表中所有数据(cat)
SELECT * FROM city;
-- 2. 查询name和population信息 (awk取列)
SELECT NAME,population FROM city;
-- 1.2.3 select+ from + where(grep)使用
-- where 配合等值查询
-- 例子:
-- 1. 查询中国所有的城市信息
SELECT * FROM city
WHERE countrycode='CHN';
-- 2. 查询ID为100的城市信息
SELECT * FROM city
WHERE id=100;
-- 3. 查询 中国河北省的城市信息
SELECT * FROM city
WHERE countrycode='CHN' AND district='hebei' ;
-- 4. 查询 中国或者美国的城市
SELECT * FROM city
WHERE countrycode='CHN' OR countrycode='USA';
或者:
SELECT * FROM city
WHERE countrycode IN ('CHN','USA');
或者:
SELECT * FROM city
WHERE countrycode='CHN'
UNION ALL
SELECT * FROM city
WHERE countrycode='USA' ;
-- where 配合范围查询
-- 例子 :
-- 1. 查询人口数量小于100人的城市
SELECT * FROM city
WHERE population<100;
-- 2. 查询人口数量100w-200w之间的
SELECT * FROM city
WHERE population>=1000000 AND population<=2000000 ;
或者:
SELECT * FROM city
WHERE population BETWEEN 1000000 AND 2000000 ;
-- 3. 查询国家代号是CH开头的城市信息
SELECT * FROM city
WHERE countrycode LIKE 'CH%';
-- 1.2.4 group by 分组子句+聚合函数应用
-- 聚合函数?
COUNT() -- 计数
SUM() -- 求和
AVG() -- 求平均值
MAX() -- 求最大值
MIN() -- 最小值
GROUP_CONCAT() -- 聚合列值
-- 结果集显示特点: 必须是1v1,不能是1vN
-- 例子 :
-- 1. 统计一下每个国家的人口总数
SELECT countrycode,SUM(population)
FROM city
GROUP BY countrycode;
-- 2. 统计中国每个省的人口总数
SELECT district,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district;
-- 3. 统计下中国每个省的城市个数及城市名.
SELECT district,COUNT(NAME),GROUP_CONCAT(NAME) FROM city
WHERE countrycode='CHN'
GROUP BY district;
-- 4. 统计每个国家城市个数
SELECT countrycode ,COUNT(NAME) FROM city
GROUP BY countrycode;
-- 1.2.6 having 后判断
-- 1. 统计中国每个省的人口总数,只显示总人口数大于500w的省信息.
SELECT district,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
HAVING SUM(population) >=5000000;
-- 1.2.7 order by 排序子句
-- 例子:
-- 1. 查询中国所有城市信息,人口数从大到小排序输出.
SELECT * FROM city
WHERE countrycode='CHN'
ORDER BY population DESC ;
-- 2. 查询中国所有城市信息,按城市名排序.
SELECT * FROM city
WHERE countrycode='CHN'
ORDER BY NAME;
-- 3. 查询中国所有省的总人口,并按总人口数从大到小排序输出.
SELECT district,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
ORDER BY SUM(population) DESC;
-- 1.2.8 limit 分页限制子句
-- 查询中国所有省的总人口,并按总人口数从大到小排序输出.
SELECT district,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
ORDER BY SUM(population) DESC
LIMIT 5 OFFSET 1;
SELECT district,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
ORDER BY SUM(population) DESC
LIMIT 10;
SELECT district,SUM(population) FROM city
WHERE countrycode='CHN'
GROUP BY district
ORDER BY SUM(population) DESC
LIMIT 1,5;
注意: LIMIT 谨慎使用, 500w+的表.
LIMIT 5000000,100
一般会改为明确查找范围
limit >xxxx and < xxxx;