【知識梳理】SQL知識梳理

distinct

select distinct country from website;

limit

 select * from website limit 2;

SQL通配符：% ,_ (代替一個字符), [charlist]

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

IN操作符（與where相關，用於嵌套查詢）： 在where中規定多個值  

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

Having操作符（與group by相關）

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

 

別名：

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：合併多個select語句結果集

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

union / union all 區別：對重複結果的處理

Select into：表數據複製到另一個表

create table xxx select * from yyy

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

• create table (...,...,unique(column_name));
• CONSTRANT xx unique(column1,column2);
• drop constrant xx
• 添加約束：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：

簡單索引：create index index_name on table_name(column_name);
唯一索引：create unique index index_name on table_name(column_name);
撤銷索引：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(類型)
刪除列：alter table table_name drop column column_name  datatype

視圖：

創建視圖：create view xx as ...  
更新視圖：create or replace view xx as...  
撤銷視圖：drop view view_name  

SQL Aggregate函數
AVG、COUNT、FIRST(第一個記錄值)、LAST(列中最後一個記錄)、MAX、MIN、GROUP BY、HAVING、UCASE(字段大寫)、LCASE（字段小寫）、MID（從文本字段中提取字段）、LEN、ROUND、FORMAT

 

典型場景：https://blog.rxliuli.com/p/73dac442/

• 行轉列：select xx max(case xx when ...... then ... else 0 end) as xxx  ............ from table group by xx
• 列轉行： union all\