数据库操作回顾

sql database基本指令

#查看数据库版本
select version();
#查看当前时间
select now()
#创建表
create database xxx charset utf8;
#使用表
use xxx
#删除表
drop database xxx;
#创建表语句
show create database xxx;
#显示所有表
show databases

sql table基本指令

#创建表
create table yyy(
id int primary key not null auto_increment,
name varchar(30)
);
#添加栏
alter table yyy add name varchar(30);
#修改字段类型
alter table yyy modify name varchar(100);
#修改名
alter table yyy change name newname varchar(20);

#删除字段
alter table yyy drop name;

#删除库
drop database xxx
#删除表
drop table xxx

sql增删改

#增加
insert into table_name values (xxx),(xxx);
#指定栏插入
insert into table_name (colum1,colum2) values (xxx),(xxx)
#修改
update table_name set xxx=xxx where xxx
#删除
delete from table_name where xxx

sql查询

#全部查询 
SELECT * from posts;

#取相应字段
SELECT title as '标题',create_time as '创建时间' FROM posts; #别名

SELECT posts.title,posts.author_id from posts;

#消除重复行
SELECT DISTINCT gender from front_user#只显示一列

#条件查询 >  <  <= >=  =  !=(或者<>) 不等于，等于，大于小于

#NOT
#查找性别不为男的用户
SELECT username,gender FROM front_user where not(gender="男") 

#AND
#查找性别不为男并且用户名不是'donghao'的用户
SELECT username,gender FROM front_user where not(gender="男" AND username="donghao")

#OR
SELECT name from album where id=1 or id=3

#模糊查询
#like  %替换一个或者多个  _替换一个
SELECT title from posts where content like "<p>用户%"

SELECT title from posts where content like "%阳光，而是微笑%"

#__
#查询至少有两个字以上的标题
SELECT title from posts where title like "__%"

#RLIKE正则
SELECT title from posts where title RLIKE "^祝.*~$"

#in
SELECT name from album where id IN (1,2,3,4,5,6,7,8)

#BETWEEN AND
SELECT name from album where id BETWEEN 1 and 100
#NOT BETWEEN AND
SELECT name from album where id not BETWEEN 1 and 100

#is null
SELECT * from front_user where realname is null;
#is not null
SELECT * from front_user where realname is not null;

排序

#从大到小
SELECT * from posts ORDER BY read_count desc
#(默认，可省略) 从小到大
SELECT * from posts ORDER BY read_count asc 

聚合函数

count  avg  max  min
SELECT COUNT(*) from posts
SELECT max(read_count) from posts
SELECT avg(read_count) from posts # or select sum(read_count)/count(read_count) from posts;
SELECT min(read_count) from posts
SELECT round(avg(read_count),2) from posts
#四舍五入 round(count,num)#保留2位小数

分组

SELECT gender,count(gender) from front_user GROUP BY gender;

#分组里面包含数据
SELECT gender,count(gender),GROUP_CONCAT(username) from front_user GROUP BY gender;

输出：

#having 过滤数据（对结果查询出的结果过滤）
SELECT gender,GROUP_CONCAT(username),avg(charactors) FROM front_user GROUP BY gender HAVING(gender!='男')

分页

select * from posts LIMIT 10
select * from posts LIMIT 1 10 #1：起始下标  10 下标

连接查询

# inner join
#只显示front_user中的nickname和posts表中的title
SELECT front_user.nickname,posts.title from front_user INNER JOIN posts ON front_user.id=posts.author_id where nickname='小姐姐'
别名：
SELECT f.nickname,p.title from front_user as f INNER JOIN posts as p ON f.id=p.author_id where nickname='小姐姐'

#left join
left join左边的表为基准，找不到置id 为null

#rightjoin
与left join相反

自关联

SELECT *FROM `comment` as c INNER JOIN `comment` as d on c.id=d.origin_comment_id where c.post_id="PuanQjvyHDzULWTj4cEick"

子查询

#查询写的字数最多的人信息
SELECT * from front_user where charactors=(SELECT MAX(charactors) from front_user)