Oracle-Join

1、建表语句

CREATE TABLE Websites(
ID INT PRIMARY KEY,
NAME VARCHAR2(30),
URL VARCHAR2(100),
alexa INT,
country VARCHAR2(30)
);
INSERT INTO Websites(ID,NAME,URL,alexa,country)VALUES(1,‘Google‘,‘http://www.google.com/‘,1,‘USA‘);
INSERT INTO Websites(ID,NAME,URL,alexa,country)VALUES(2,‘淘宝‘,‘http://www.taobao.com/‘,13,‘CN‘);
INSERT INTO Websites(ID,NAME,URL,alexa,country)VALUES(3,‘天猫‘,‘http://www.tianmao.com/‘,16,‘CN‘);
INSERT INTO Websites(ID,NAME,URL,alexa,country)VALUES(4,‘微博‘,‘http://www.weibo.com/‘,20,‘CN‘);
INSERT INTO Websites(ID,NAME,URL,alexa,country)VALUES(5,‘Facebook‘,‘http://www.facebook.com/‘,3,‘USA‘);
INSERT INTO Websites(ID,NAME,URL,alexa,country)VALUES(7,‘stackoverflow‘,‘http://www.stackoverflow.com/‘,0,‘IND‘);
CREATE TABLE access_log(
aid INT PRIMARY KEY,
site_id INT,
COUNT INT,
datetime date
);
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(1,1,45,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(2,3,100,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(3,1,230,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(4,2,10,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(5,5,205,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(6,4,13,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(7,3,220,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(8,5,545,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
INSERT INTO access_log(aid,site_id,COUNT,datetime)VALUES(9,3,201,to_timestamp(to_char(systimestamp,‘YYYY-MM-DD HH24:MI:SS.FF‘),‘YYYY-MM-DD HH24:MI:SS.FF‘));
SELECT * FROM Websites;
SELECT * FROM access_log;

2、SQL 连接(JOIN)

SQL join 用于把来自两个或多个表的行结合起来。

下图展示了 LEFT JOIN、RIGHT JOIN、INNER JOIN、OUTER JOIN 相关的 7 种用法。

（1） INNER JOIN从多个表中返回满足 JOIN 条件的所有行

SELECT 
  w.id,
  w.name,
  w.url,
  w.alexa,
  w.country,
  a.aid,
  a.site_id,
  a.count,
  a.datetime
FROM Websites w
INNER JOIN access_log a
ON w.id = a.site_id

     不同的 SQL JOIN

• INNER JOIN：如果表中有至少一个匹配，则返回行
• LEFT JOIN：即使右表中没有匹配，也从左表返回所有的行
• RIGHT JOIN：即使左表中没有匹配，也从右表返回所有的行
• FULL JOIN：只要其中一个表中存在匹配，则返回行

   （2）LEFT JOIN

Oracle-Join

原文：https://www.cnblogs.com/hongwei2085/p/10808225.html