【知识梳理】SQL知识梳理

distinct

select distinct country from website;

limit

 select * from website limit 2;

SQL通配符：% ,_ (代替一个字符), [charlist]

select * from website where name like '_oogle'
select * from website where name REGEXP '^[GFS]'

IN操作符（与where相关，用于嵌套查询）： 在where中规定多个值  

select * from website where name in ('Google','菜鸟')

Having操作符（与group by相关）

嵌套查询：https://www.cnblogs.com/shengulong/p/10217146.html

 

别名：

select cloumn_name as alias_name;
select name,CONCAT(url,',',country) as site_info from website;

JOIN...ON： inner join、left join、right join、full  join

select * from A inner join B on A.xx = B.xx

UNION：合并多个select语句结果集

select country from A
union
select country from B
order by country;

union / union all 区别：对重复结果的处理

Select into：表数据复制到另一个表

create table xxx select * from yyy

约束：NOT NULL、UNIQUE、PRIMARY KEY、FOREIGN KEY、CHECK、DEFAULT

• create table (...,...,unique(column_name));
• CONSTRANT xx unique(column1,column2);
• drop constrant xx
• 添加约束：alter table xxx add unique(column_name) drop unique ... primary key ...
• foreign key (column_name) references A(xx)
• check(column >0)
• alter table A alter column B set default 'xxx'

索引：

简单索引：create index index_name on table_name(column_name);
唯一索引：create unique index index_name on table_name(column_name);
撤销索引：drop index table_name.index_name

修改：

修改表名：alter table table_name rename to xxx
增加列：alter table table_name alter column column_name datatype(类型)
删除列：alter table table_name drop column column_name  datatype

视图：

创建视图：create view xx as ...  
更新视图：create or replace view xx as...  
撤销视图：drop view view_name  

SQL Aggregate函数
AVG、COUNT、FIRST(第一个记录值)、LAST(列中最后一个记录)、MAX、MIN、GROUP BY、HAVING、UCASE(字段大写)、LCASE（字段小写）、MID（从文本字段中提取字段）、LEN、ROUND、FORMAT

 

典型场景：https://blog.rxliuli.com/p/73dac442/

• 行转列：select xx max(case xx when ...... then ... else 0 end) as xxx  ............ from table group by xx
• 列转行： union all\