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MySQL数据查询之连接查询

MySQL数据查询之连接查询

  • 连接关系是关系数据库模型的主要特点。也是最主要的查询,主要包括内连接、外连接等。
  • 通过连接运算符可以实现多个表查询。
  • 在关系数据库管理系统中,表建立时各数据之间的关系不必确定,常把一个实体的所有信息存放在一个表中。当查询数据时,通过连接操作查询出存放在多个表中不同实体的信息。当两个或多个表中存在相同意义的字段时,便可以通过这些字段对不同的表进行连接查询。

下面介绍多表之间的三种连接方式:

(1)内连接查询
(2)外连接查询
(3)复合条件连接查询

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(1)、内连接查询(inner join)
  • 内连接使用比较运算符进行表间某(某些)列数据的比较操作,并列出这些表中与连接条件向匹配的数据行,组成新纪录,也就是说,在内连接查询中,只有满足条件的记录才能出现在结果关系中。

下面创建数据表suppliers进行演示:

mysql> 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 (0.17 sec)mysql> 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','Taiyuan','030000','22222'),
    -> (106,'Just Eat Ours','Beijing','010','45678'),
    -> (107,'DK Inc','Zhengzhou','450000','33332');Query OK, 7 rows affected (0.07 sec)Records: 7  Duplicates: 0  Warnings: 0

【例1】在fruits表和suppliers表之间使用内连接查询。

mysql> 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 (0.06 sec)mysql> 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 (0.00 sec)

可以看到fruits和suppliers表中都有相同数据类型的字段s_id,两个表通过s_id字段建立联系。

接下来从fruits表中查询f_name、f_price字段,从suppliers表中查询s_id、s_name,SQL语句如下:

mysql> select suppliers.s_id,s_name,f_name,f_price    -> from fruits,suppliers    -> where fruits.s_id=suppliers.s_id;+------+----------------+------------+---------+| s_id | s_name         | f_name     | f_price |+------+----------------+------------+---------+|  104 | FNK Inc.       | lemon      |    6.40 ||  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 ||  106 | Just Eat Ours  | mango      |   15.70 ||  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         | xbabay     |    3.60 |+------+----------------+------------+---------+16 rows in set (0.00 sec)
  • 注意:因为fruits表和suppliers表中有相同的字段s_id,因此在比较的时候,需要完全限定表名,格式为"表名.列名",如果只给出s_id,MySQL将不知道指的是哪一个,并返回错误信息。

【例2】在fruits表和suppliers表之间,使用inner join语法进行内连接查询,SQL语句如下:

mysql> select suppliers.s_id,s_name,f_name,f_price    -> from fruits inner join suppliers    -> on fruits.s_id = suppliers.s_id;+------+----------------+------------+---------+| s_id | s_name         | f_name     | f_price |+------+----------------+------------+---------+|  104 | FNK Inc.       | lemon      |    6.40 ||  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 ||  106 | Just Eat Ours  | mango      |   15.70 ||  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         | xbabay     |    3.60 |+------+----------------+------------+---------+16 rows in set (0.00 sec)
  • 使用where子句定义连接条件比较简单明了,而inner join语法是ANSI SQL 的标准规范,使用inner join连接语法能够确保不会忘记连接条件,而且,where子句在某些时候回影响查询的性能。

如果在一个连接查询中涉及的两个表都是同一个表,name这种查询称为自连接查询。自连接是一种特殊的内连接,是指相互连接的表在物理上为同一张表,但可以在逻辑上分为两张表。

【例3】查询供应f_id='a1'的水果供应商提供的水果种类,SQL语句如下:

mysql> select f1.f_id,f1.f_name    -> from fruits as f1,fruits as f2    -> where f1.s_id =f2.s_id and f2.f_id ='a1';+------+------------+| f_id | f_name     |+------+------------+| a1   | apple      || b1   | blackberry || c0   | cherry     |+------+------------+3 rows in set (0.00 sec)

(2)、外连接查询
  • 外连接查询将查询多个表中相关联的行。
  • 内连接时,返回查询结果集合中的仅是符合查询条件和连接条件的行。但有时候需要包含没有关联的行中数据,即返回查询结果集合中的不仅包含符合连接条件的行,而且还包括左表(左外连接或左连接)、右表(右外连接或右连接)或两个边接表(全外连接)中的所有数据行。外连接分为左外连接或左连接和右外连接或右连接:

①左连接:返回包括左表中的所有记录和右表中连接字段相等的记录。
②右连接:返回包括右表中的记录和左表中连接字段相等的记录。

1.左连接left join

首先创建表orders,SQL语句如下;

mysql> 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 (0.11 sec)mysql> 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 (0.06 sec)Records: 5  Duplicates: 0  Warnings: 0

【例】在customers表和orders表中,查询所有客户,包括没有订单的客户,SQL语句如下:

mysql> select * from customers;+-------+----------+---------------------+---------+--------+-----------+-------------------+| c_id  | c_name   | c_address           | c_city  | c_zip  | c_contact | c_email           |+-------+----------+---------------------+---------+--------+-----------+-------------------+| 10001 | redhool  | 200 Street          | Tianjin | 300000 | LiMing    | LMing@163.com     || 10002 | Stars    | 333 Fromage Lane    | Dalian  | 116000 | Zhangbo   | Jerry@hotmail.com || 10003 | Netbhood | 1 Sunny Place       | Qingdao | 266000 | LuoCong   | NULL              || 10004 | JOTO     | 829 Riverside Drive | Haikou  | 570000 | YangShan  | sam@hotmail.com   |+-------+----------+---------------------+---------+--------+-----------+-------------------+4 rows in set (0.00 sec)mysql> select * from orders;+-------+---------------------+-------+| o_num | o_date              | c_id  |+-------+---------------------+-------+| 30001 | 2008-09-01 00:00:00 | 10001 || 30002 | 2008-09-12 00:00:00 | 10003 || 30003 | 2008-09-30 00:00:00 | 10004 || 30004 | 2008-10-03 00:00:00 | 10005 || 30005 | 2008-10-08 00:00:00 | 10001 |+-------+---------------------+-------+5 rows in set (0.00 sec)mysql>  select customers.c_id , orders.o_num    -> from customers left outer join orders    -> on customers.c_id = orders.c_id;+-------+-------+| c_id  | o_num |+-------+-------+| 10001 | 30001 || 10003 | 30002 || 10004 | 30003 || 10001 | 30005 || 10002 |  NULL |+-------+-------+5 rows in set (0.00 sec)

2.右连接right join

  • 右连接是左连接的反向连接,将返回右表的所有行。如果右表的某行在左表中没有匹配行,左表将返回空值。

【例】在customers表和orders表中,查询所有订单,包括没有客户的订单,SQL语句如下:

mysql> select customers.c_id, orders.o_num    -> from customers right outer join orders    -> on customers.c_id = orders.c_id;+-------+-------+| c_id  | o_num |+-------+-------+| 10001 | 30001 || 10003 | 30002 || 10004 | 30003 ||  NULL | 30004 || 10001 | 30005 |+-------+-------+5 rows in set (0.00 sec)

(3)、复合条件连接查询
  • 复合条件连接查询是在连接查询的过程中,通过添加过滤条件,限制查询结果,使查询的结果更加准确。

【例1】在customers表和orders表中,使用inner join语法查询customers表汇总ID为10001的客户的订单信息,SQL语句如下:

mysql> select customers.c_id,orders.o_num    -> from customers inner join orders    -> on customers.c_id = orders.c_id and customers.c_id = 10001;+-------+-------+| c_id  | o_num |+-------+-------+| 10001 | 30001 || 10001 | 30005 |+-------+-------+2 rows in set (0.00 sec)

【例2】在fruits表和suppliers表之间,使用inner join语法进行内连接查询,并对查询结果排序,SQL语句如下:

mysql> select suppliers.s_id,s_name,f_name,f_price    -> from fruits inner join suppliers    -> on fruits.s_id = suppliers.s_id    -> order by fruits.s_id;+------+----------------+------------+---------+| 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    | orange     |   11.20 ||  102 | LT Supplies    | banana     |   10.30 ||  102 | LT Supplies    | grape      |    5.30 ||  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       | melon      |    8.20 ||  105 | Good Set       | xbabay     |    2.60 ||  105 | Good Set       | xxtt       |   11.60 ||  106 | Just Eat Ours  | mango      |   15.70 ||  107 | DK Inc         | xxxx       |    3.60 ||  107 | DK Inc         | xbabay     |    3.60 |+------+----------------+------------+---------+16 rows in set (0.00 sec)

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