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第一:内联(inner join)
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如果想把用户信息、积分、等级都列出来,那么一般会这样写:
select * from T1, T3 where T1.userid = T3.userid
(其实这样的结果等同于select * from T1 inner join T3 on T1.userid=T3.userid )。
把两个表中都存在userid的行拼成一行(即内联),但后者的效率会比前者高很多,建议用后者(内联)的写法。
SQL语句:
select * from T1 inner join T2 on T1.userid = T2.userid
运行结果
T1.userid username password T2.userid jifen dengji
1 jack jackpwd 1 20 3
第二:左联(left outer join)
显示左表T1中的所有行,并把右表T2中符合条件加到左表T1中;
右表T2中不符合条件,就不用加入结果表中,并且NULL表示。
SQL语句:
select * from T1 left outer join T2 on T1.userid = T2.userid
运行结果
T1.userid username password T2.userid jifen dengji
1 jack jackpwd 1 20 3
2 owen owenpwd NULL NULL NULL
第三:右联(right outer join)。
显示右表T2中的所有行,并把左表T1中符合条件加到右表T2中;
左表T1中不符合条件,就不用加入结果表中,并且NULL表示。
SQL语句:
select * from T1 right outer join T2 on T1.userid = T2.userid
运行结果
T1.userid username password T2.userid jifen dengji
1 jack jackpwd 1 20 3
NULL NULL NULL 3 50 6
第四:全联(full outer join)
显示左表T1、右表T2两边中的所有行,即把左联结果表 + 右联结果表组合在一起,然后过滤掉重复的。
SQL语句:
select * from T1 full outer join T2 on T1.userid = T2.userid
方法和操作步骤如下:
1、首先,创建一个测试表,如下图所示,然后进入下一步。
2、其次,插入测试数据,如下图所示,然后进入下一步。
3、接着,完成上述步骤后,查询表中的数据,“select t.* from test_tbl2 t ”,如下图所示,然后进入下一步。
4、最后,完成上述步骤后,编写sql,两个表通过pid与id关联, “select t1.*, t2.* from test_tbl1 t1 join test_tbl2 t2 on t1.p_id = t2.id;”,如下图所示。这样,问题就解决了。
1.新建表
create table websites(id int not null, name varchar(32), url varchar(100) not null ,alexa int not null , country varchar(32));
插入数据
insert into websites values(1,'Google',' ',1,'USA' );
insert into websites values(2,'taobao',' ',13,'CN' );
insert into websites values(3,'cainiao',' ',4689,'CN' );
insert into websites values(4,'weibo',' ',20,'CN' );
insert into websites values(5,'Facebook',' ',3,'USA' );
insert into websites values(7,'stackoverflow',' ',0,'IND' );
create table access_log(aid int not null, site_id int not null, count int, date varchar(100));
insert into access_log values(1,1,45,'2016-05-10');
insert into access_log values(2,3,100,'2016-05-13');
insert into access_log values(3,1,23,'2016-05-14');
insert into access_log values(4,2,10,'2016-05-14');
insert into access_log values(5,5,205,'2016-05-14');
insert into access_log values(6,4,13,'2016-05-15');
insert into access_log values(7,3,220,'2016-05-15');
insert into access_log values(8,5,545,'2016-05-16');
insert into access_log values(9,3,201,'2016-05-17');
insert into access_log values(10,6,111,'2016-03-19');
select * from mysql.test.websites
| 1 | Google | | 1 | USA |
| 2 | 淘宝 | | 13 | CN |
| 3 | 菜鸟教程 | | 4689 | CN |
| 4 | 微博 | | 20 | CN |
| 5 | Facebook | | 3 | USA |
| 7 | stackoverflow | | 0 | IND |
+-----+---------+-------+------------+
| aid | site_id | count | date |
+-----+---------+-------+------------+
| 1 | 1 | 45 | 2016-05-10 |
| 2 | 3 | 100 | 2016-05-13 |
| 3 | 1 | 230 | 2016-05-14 |
| 4 | 2 | 10 | 2016-05-14 |
| 5 | 5 | 205 | 2016-05-14 |
| 6 | 4 | 13 | 2016-05-15 |
| 7 | 3 | 220 | 2016-05-15 |
| 8 | 5 | 545 | 2016-05-16 |
| 9 | 3 | 201 | 2016-05-17 |
+-----+---------+-------+------------+
2.关联查询
内连接
SELECT *
FROM Websites
INNER JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY Websites.id;
左连接
SELECT Websites.name, access_log.count, access_log.date
FROM Websites
LEFT JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count DESC;
右连接
SELECT Websites.name, access_log.count, access_log.date
FROM Websites
RIGHT JOIN access_log
ON Websites.id=access_log.site_id
ORDER BY access_log.count DESC;
全连接
SELECT websites.name, access_log.count, access_log.date
FROM websites
FULL OUTER JOIN access_log
ON access_log.site_id=websites.id
ORDER BY access_log.count DESC;
mysql
两个表中的信息关联起来使用方法:
1、创建主表:
create
table
UserInfo(
UserID
int
identity(1,1)
primary
key,
--递增主键
UserAccounts
varchar(20),
UserName
varchar(20),
UserPwd
varchar(10));
2、创建附表(含外键)
create
table
News(
NewsID
int
identity(1,1)
primarykey,
UserID
int,
NewsTitle
varchar(
50
),
NewsRelease
varchar(
200
),
NewsReleaseTime
datetime,
FOREIGN
KEY
(UserID)
REFERENCES
UserInfo(UserID));
--外键约束
如果附表已存在,但没外键,可采用以下方法:
alter
table
profession
add
constraint
fk_prov_id
foreign
key(prov_id)
references
province(prov_id)
on
update
cascade
on
delete
cascade;