连接查询：

1、连接查询分为内连接和外连接，其中外连接又分为左连接和右连接。

2、内连接：使用比较运算符进行表面某列数据的的比较操作，并列出这些表中与连接条件相匹配的数据行。只返回满足条件的记录。

3、外连接：返回的结果不仅包括符合条件的行，还包括不符合条件的行

（左外连接）：返回包括左表中的所有记录和游标中连接字段相等的记录。结果包括LEFT OUTER子句中指定的左表所有行。

（右外连接）：返回包括右表中的所有记录和左表中连接字段相等的记录。是左连接的反向连接，将返回右表的所有行。

1、准备数据表

#创建名为suppliers的数据表
MariaDB [vincen]> CREATE TABLE suppliers
    -> (
    ->   s_id      int      NOT NULL AUTO_INCREMENT,
    ->   s_name    char(50) NOT NULL,
    ->   s_city    char(50) NULL,
    ->   s_zip     char(10) NULL,
    ->   s_call    CHAR(50) NOT NULL,
    ->   PRIMARY KEY (s_id)
    -> ) ;
Query OK, 0 rows affected (0.02 sec)
#为suppliers数据表每个相应的字段插入数据
MariaDB [vincen]> INSERT INTO suppliers(s_id, s_name,s_city,  s_zip, s_call)
    -> VALUES(101,'FastFruit Inc.','Tianjin','300000','48075'),
    -> (102,'LT Supplies','Chongqing','400000','44333'),
    -> (103,'ACME','Shanghai','200000','90046'),
    -> (104,'FNK Inc.','Zhongshan','528437','11111'),
    -> (105,'Good Set','Taiyuang','030000', '22222'),
    -> (106,'Just Eat Ours','Beijing','010', '45678'),
    -> (107,'DK Inc.','Zhengzhou','450000', '33332');
Query OK, 7 rows affected (0.01 sec)
Records: 7  Duplicates: 0  Warnings: 0
#创建名为orders的数据表
MariaDB [vincen]> CREATE TABLE orders
    -> (
    ->   o_num  int      NOT NULL AUTO_INCREMENT,
    ->   o_date datetime NOT NULL,
    ->   c_id   int      NOT NULL,
    ->   PRIMARY KEY (o_num)
    -> ) ;
Query OK, 0 rows affected (0.02 sec)
#给orders数据表中每个字段插入数据
MariaDB [vincen]> INSERT INTO orders(o_num, o_date, c_id)
    -> VALUES(30001, '2008-09-01', 10001),
    -> (30002, '2008-09-12', 10003),
    -> (30003, '2008-09-30', 10004),
    -> (30004, '2008-10-03', 10005),
    -> (30005, '2008-10-08', 10001);
Query OK, 5 rows affected (0.01 sec)
Records: 5  Duplicates: 0  Warnings: 0
#创建名为customers的数据表
MariaDB [vincen]> CREATE TABLE customers
    -> (
    ->   c_id      int       NOT NULL AUTO_INCREMENT,
    ->   c_name    char(50)  NOT NULL,
    ->   c_address char(50)  NULL,
    ->   c_city    char(50)  NULL,
    ->   c_zip     char(10)  NULL,
    ->   c_contact char(50)  NULL,
    ->   c_email   char(255) NULL,
    ->   PRIMARY KEY (c_id)
    -> );
Query OK, 0 rows affected (0.01 sec)
#给customers数据表中每个字段插入数据
MariaDB [vincen]> INSERT INTO customers(c_id, c_name, c_address, c_city, 
    -> c_zip,  c_contact, c_email) 
    -> VALUES(10001, 'RedHook', '200 Street ', 'Tianjin', 
    ->  '300000',  'LiMing', '[email protected]'),
    -> (10002, 'Stars', '333 Fromage Lane',
    ->  'Dalian', '116000',  'Zhangbo','[email protected]'),
    -> (10003, 'Netbhood', '1 Sunny Place', 'Qingdao',  '266000',
    ->  'LuoCong', NULL),
    -> (10004, 'JOTO', '829 Riverside Drive', 'Haikou', 
    ->  '570000',  'YangShan', '[email protected]');
Query OK, 4 rows affected (0.01 sec)
Records: 4  Duplicates: 0  Warnings: 0

2、查看两个数据表的结构

MariaDB [vincen]> desc fruits;
+---------+--------------+------+-----+---------+-------+
| Field   | Type         | Null | Key | Default | Extra |
+---------+--------------+------+-----+---------+-------+
| f_id    | char(10)     | NO   | PRI | NULL    |       |
| s_id    | int(11)      | NO   |     | NULL    |       |
| f_name  | char(255)    | NO   |     | NULL    |       |
| f_price | decimal(8,2) | NO   |     | NULL    |       |
+---------+--------------+------+-----+---------+-------+
4 rows in set (0.01 sec)

MariaDB [vincen]> desc suppliers;
+--------+----------+------+-----+---------+----------------+
| Field  | Type     | Null | Key | Default | Extra          |
+--------+----------+------+-----+---------+----------------+
| s_id   | int(11)  | NO   | PRI | NULL    | auto_increment |
| s_name | char(50) | NO   |     | NULL    |                |
| s_city | char(50) | YES  |     | NULL    |                |
| s_zip  | char(10) | YES  |     | NULL    |                |
| s_call | char(50) | NO   |     | NULL    |                |
+--------+----------+------+-----+---------+----------------+
5 rows in set (0.01 sec)

3、使用INNER JOIN内连接

#在fruits表和suppliers表中使用内连接查询
MariaDB [vincen]> SELECT suppliers.s_id,s_name   #查询suppliers表中的s_id,s_name字段
    -> f_name,f_price                            #查询fruits表中的f_name,f_price字段
    -> FROM fruits INNER JOIN suppliers          #从fruits内连接suppliers的表中
    -> ON fruits.s_id = suppliers.s_id;  #条件是fruits表中的s_id字段和suppliers表中的s_id字段相等
+------+----------------+------------+---------+
| s_id | s_name         | f_name     | f_price |
+------+----------------+------------+---------+
|  101 | FastFruit Inc. | apple      |    5.20 |
|  103 | ACME           | apricot    |    2.20 |
|  101 | FastFruit Inc. | blackberry |   10.20 |
|  104 | FNK Inc.       | berry      |    7.60 |
|  107 | DK Inc.        | xxxx       |    3.60 |
|  102 | LT Supplies    | orange     |   11.20 |
|  105 | Good Set       | melon      |    8.20 |
|  101 | FastFruit Inc. | cherry     |    3.20 |
|  104 | FNK Inc.       | lemon      |    6.40 |
|  106 | Just Eat Ours  | mango      |   15.60 |
|  105 | Good Set       | xbabay     |    2.60 |
|  105 | Good Set       | xxtt       |   11.60 |
|  103 | ACME           | coconut    |    9.20 |
|  102 | LT Supplies    | banana     |   10.30 |
|  102 | LT Supplies    | grape      |    5.30 |
|  107 | DK Inc.        | xbababa    |    3.60 |
+------+----------------+------------+---------+
16 rows in set (0.01 sec)

4、左外连接查询（LEFT JOIN）

#在customers表和orders表中，查询所有客户，包括没有订单的用户
MariaDB [vincen]>  SELECT customers.c_id,     #查询customers表中的c_id字段 
    -> orders.o_num                           #查询orders表中的o_num字段
    -> FROM customers LEFT OUTER JOIN orders  #customers作为左表外连接右表orders
    -> ON                                     #条件是
    -> customers.c_id = orders.c_id;       #customers表中的c_id字段和orders表中的c_id字段相等
+-------+-------+
| c_id  | o_num |
+-------+-------+
| 10001 | 30001 |
| 10003 | 30002 |
| 10004 | 30003 |
| 10001 | 30005 |
| 10002 |  NULL |          #ID等于1002的客户并没有下单，所以对应的orders表中没有该用户的信息
+-------+-------+
5 rows in set (0.02 sec)

5、右外连接查询（RIGHT JOIN）

#在customers表和orders表中，查询所有订单，包括没有客户的订单
MariaDB [vincen]> SELECT customers.c_id,       #查询customers表中的c_id字段
    -> orders.o_num                            #查询orders表中的o_num字段
    -> FROM customers RIGHT OUTER JOIN orders  #customers表作为右表外连接orders表
    -> ON                                      #条件是
    -> customers.c_id = orders.c_id;      #customers表中的c_id字段和orders表中的c_id字段相等
+-------+-------+
| c_id  | o_num |
+-------+-------+
| 10001 | 30001 |
| 10003 | 30002 |
| 10004 | 30003 |
|  NULL | 30004 |
| 10001 | 30005 |
+-------+-------+
5 rows in set (0.01 sec)

6、复合条件连接查询

在连接查询的过程中，通过添加过滤条件，限制查询的结果，使查询的结果更加准确。

#在fruits表和suppliers表之间，使用INNER JOIN语法进行内连接查询，并对结果排序
MariaDB [vincen]> SELECT suppliers.s_id,    #查询suppliers表中的s_id字段
    -> s_name,                              #查询suppliers表中的s_name字段
    -> f_name, f_price                      #查询fruits表中的f_name和f_price字段
    -> FROM fruits INNER JOIN suppliers     #fruits表内连接suppliers表
    -> ON                                   #条件是
    -> fruits.s_id = suppliers.s_id      #fruits表中的s_id字段和suppliers表中的s_id字段相等
    -> ORDER BY fruits.s_id;                #根据fruits表中的s_id字段排序
+------+----------------+------------+---------+
| s_id | s_name         | f_name     | f_price |
+------+----------------+------------+---------+
|  101 | FastFruit Inc. | apple      |    5.20 |
|  101 | FastFruit Inc. | blackberry |   10.20 |
|  101 | FastFruit Inc. | cherry     |    3.20 |
|  102 | LT Supplies    | grape      |    5.30 |
|  102 | LT Supplies    | banana     |   10.30 |
|  102 | LT Supplies    | orange     |   11.20 |
|  103 | ACME           | apricot    |    2.20 |
|  103 | ACME           | coconut    |    9.20 |
|  104 | FNK Inc.       | lemon      |    6.40 |
|  104 | FNK Inc.       | berry      |    7.60 |
|  105 | Good Set       | xbabay     |    2.60 |
|  105 | Good Set       | xxtt       |   11.60 |
|  105 | Good Set       | melon      |    8.20 |
|  106 | Just Eat Ours  | mango      |   15.60 |
|  107 | DK Inc.        | xxxx       |    3.60 |
|  107 | DK Inc.        | xbababa    |    3.60 |
+------+----------------+------------+---------+
16 rows in set (0.01 sec)

连接查询MySQL数据

连接查询：

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