作用：
# 多表聯查，聯表查詢

1.傳統連線

1.集合
#集合
[xiaoqiu,xiaowang,qiandao]
[80,90,100]

#資料庫
id:[1,2,3]
name:[xiaoqiu,xiaowang,qiandao]

id:[1,2,3]
mark:[80,90,100]

2.建表
mysql> create table students(id int,name varchar(10));
Query OK, 0 rows affected (0.08 sec)

mysql> create table score(id int,mark int);
Query OK, 0 rows affected (0.05 sec)

3.插入資料
mysql> insert into students values(1,'xiaoqiu'),(2,'qianqian'),(3,'xiaowang');
Query OK, 3 rows affected (0.02 sec)
Records: 3  Duplicates: 0  Warnings: 0

mysql> insert into score values(1,80),(2,90),(3,100);
Query OK, 3 rows affected (0.01 sec)
Records: 3  Duplicates: 0  Warnings: 0

4.資料查詢
# 檢視兩個表的資料
mysql> select * from students;
+------+---------+
| id   | name    |
+------+---------+
|    1 | xiaoqiu |
|    2 | qiandao |
|    3 | xiaowang|
+------+---------+
3 rows in set (0.00 sec)

mysql> select * from score;
+------+------+
| id   | mark |
+------+------+
|    1 |   80 |
|    2 |   90 |
|    3 |  100 |
+------+------+
3 rows in set (0.00 sec)

# 檢視xiaoqiu的分數
mysql> select students.name,score.mark from students,score where students.id=1 and score.id=1;

mysql> select students.name,score.mark from students,score where students.id=score.id and name='xiaoqiu';
+--------+------+
| name   | mark |
+--------+------+
| xiaoqiu |   80 |
+--------+------+
1 row in set (0.01 sec)

# 查詢所有學生成績
mysql> select students.name,score.mark from students,score where students.id=score.id

5.連表查詢：世界上小於100人的城市在哪個國家？請列出城市名字，國家名字與人口數量
# 1.確認我要查哪些內容
國家名字  城市名字  城市人口數量   小於100人

# 2.確認在哪個表
country.name   city.name   city.population   

# 3.找出兩個表相關聯的欄位
city.countrycode   country.code

# 4.編寫語句
mysql> select country.name,city.name,city.population from country,city where city.countrycode=country.code and city.population < 100;
+----------+-----------+------------+
| name     | name      | population |
+----------+-----------+------------+
| Pitcairn | Adamstown |         42 |
+----------+-----------+------------+
1 row in set (0.01 sec)

6.練習題二：連表查詢：世界上小於100人的城市在哪個國家，是用什麼語言？請列出城市名字，國家名字與人口數量和國家語言
# 1.確認我要查哪些內容
國家名字  城市名字  城市人口數量   國家使用的語言   小於100人

# 2.確認在哪個表
country.name   city.name   city.population   countrylanguage.language

# 3.找出三個表相關聯的欄位
country.code   city.countrycode   countrylanguage.countrycode

# 4.寫sql語句
mysql> select country.name,city.name,city.population,countrylanguage.language from country,city,countrylanguage where country.code=city.countrycode and city.countrycode=countrylanguage.countrycode and city.population < 100;
+----------+-----------+------------+-------------+
| name     | name      | population | language    |
+----------+-----------+------------+-------------+
| Pitcairn | Adamstown |         42 | Pitcairnese |
+----------+-----------+------------+-------------+
1 row in set (0.04 sec)
 

2.自連線

#自己查詢相同欄位，使用自連線，兩個關聯的表必須有相同欄位和相同資料
SELECT city.name,city.countrycode,countrylanguage.language,city.population
FROM  city NATURAL JOIN countrylanguage 
WHERE population > 1000000
ORDER BY population;

#兩個表中沒有相同欄位不行，欄位相同值不同不行
SELECT country.name,city.name,city.population FROM city NATURAL JOIN country WHERE population < 100;

#注意：
1.自連線必須有相同欄位和相同值
2.兩個表中的資料必須完全相同
 

3.內連線

1.語法格式

select * from 表1 join 表2 on 相關聯的條件 where 條件;
#注意：命中率（驅動的概念）
	表1 小表
	表2 大表
	
select * from 表1 inner join 表2 on 相關聯的條件 where 條件;

2.例子1：兩表聯查

#小於100人的城市在哪個國家，國家程式碼是什麼？
select city.name,city.population,city.countrycode,country.name 
from city join country on city.countrycode=country.code 
where city.population < 100;
 

3.例子2：三表聯查

#世界上小於100人的城市在哪個國家？是用什麼語言？
select country.name,city.name,city.population,countrylanguage.language
from city join country on city.countrycode=country.code 
join countrylanguage on country.code=countrylanguage.countrycode
where city.population < 100;

4.外連線（有問題）

1.左外連線

select city.name,city.countrycode,country.name,city.population
from city left join country 
on city.countrycode=country.code 
and city.population < 100 limit 5;
+----------------+-------------+------+------------+
| name           | countrycode | name | population |
+----------------+-------------+------+------------+
| Kabul          | AFG         | NULL |    1780000 |
| Qandahar       | AFG         | NULL |     237500 |
| Herat          | AFG         | NULL |     186800 |
| Mazar-e-Sharif | AFG         | NULL |     127800 |
| Amsterdam      | NLD         | NULL |     731200 |
+----------------+-------------+------+------------+

2.右外連線

select city.name,city.countrycode,country.name,city.population
from city right join country 
on city.countrycode=country.code
and city.population < 100 limit 5;
+------+-------------+-------------+------------+
| name | countrycode | name        | population |
+------+-------------+-------------+------------+
| NULL | NULL        | Aruba       |       NULL |
| NULL | NULL        | Afghanistan |       NULL |
| NULL | NULL        | Angola      |       NULL |
| NULL | NULL        | Anguilla    |       NULL |
| NULL | NULL        | Albania     |       NULL |
+------+-------------+-------------+------------+