超详细mysql left join,right join,inner join用法分析
2017-09-27 19:34
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超详细mysql left join,right join,inner join用法分析
mysql> show create table EMPLOYEE \G;
*************************** 1. row ***************************
Table: EMPLOYEE
Create Table: CREATE TABLE `EMPLOYEE` (
`FIRST_NAME` char(20) NOT NULL,
`LAST_NAME` char(20) DEFAULT NULL,
`AGE` int(11) DEFAULT NULL,
`SEX` char(1) DEFAULT NULL,
`INCOME` float DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
(1)创建并插入数据,结果如下:
CREATE TABLE `a` (
`a_id` int(11)
AUTO_INCREMENT PRIMARY KEY,
`a_id_code` char(25) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `b` (
`b_id` int(11)
AUTO_INCREMENT PRIMARY KEY,
`b_id_code` char(25) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO a
VALUES ( 1, 'a20050111' ) , ( 2, 'a20050112' ) , ( 3, 'a20050113' ) , ( 4, 'a20050114' ) , ( 5, 'a20050115' ) ;
INSERT INTO b
VALUES ( 1, '2006032401' ) , ( 2, '2006032402' ) , ( 3, '2006032403' ) , ( 4, '2006032404' ) , ( 8, '2006032408' ) ;
结果如下:
mysql> select * from a;
| a_id | a_id_code |
+------+-----------+
| 1 | a20050111 |
| 2 | a20050112 |
| 3 | a20050113 |
| 4 | a20050114 |
| 5 | a20050115 |
+------+-----------+
5 rows in set (0.00 sec)
mysql> select * from b;
+------+------------+
| b_id | b_id_code |
+------+------------+
| 1 | 2006032401 |
| 2 | 2006032402 |
| 3 | 2006032403 |
| 4 | 2006032404 |
| 8 | 2006032408 |
+------+------------+
5 rows in set (0.01 sec)
(2)left join(左联接)
mysql> SELECT * FROM a LEFT JOIN b on a.a_id=b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | 1 | 2006032401 |
| 2 | a20050112 | 2 | 2006032402 |
| 3 | a20050113 | 3 | 2006032403 |
| 4 | a20050114 | 4 | 2006032404 |
| 5 | a20050115 | NULL | NULL |
+------+-----------+------+------------+
5 rows in set (0.00 sec)
结果说明:
left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,left join是以左表为准的.
换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录(例子中为: A.aID = B.bID).
B表记录不足的地方均为NULL.
(3)right join(右联接)
mysql> SELECT * FROM a right JOIN b on a.a_id=b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | 1 | 2006032401 |
| 2 | a20050112 | 2 | 2006032402 |
| 3 | a20050113 | 3 | 2006032403 |
| 4 | a20050114 | 4 | 2006032404 |
| NULL | NULL | 8 | 2006032408 |
+------+-----------+------+——————+
结果说明:
仔细观察一下,就会发现,和left join的结果刚好相反,这次是以右表(B)为基础的,A表不足的地方用NULL填充.
(4) inner join(相等联接或内联接)
mysql> SELECT * FROM a inner JOIN b on a.a_id=b.b_id;
等价于
mysql> SELECT * FROM a, b WHERE a.a_id=b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | 1 | 2006032401 |
| 2 | a20050112 | 2 | 2006032402 |
| 3 | a20050113 | 3 | 2006032403 |
| 4 | a20050114 | 4 | 2006032404 |
+------+-----------+------+------------+
4 rows in set (0.00 sec)
结果说明:
很明显,这里只显示出了 A.aID = B.bID的记录.这说明inner join并不以谁为基础,它只显示符合条件的记录.
LEFT JOIN操作用于在任何的 FROM 子句中, 组合来源表的记录。使用 LEFT JOIN 运算来创建一个左边外部联接。左边外部联接将包含了从第一个(左边)开始的两个表中的全部记录,即 使在第二个(右边)表中并没有相符值的记录。
(5)
语法:FROM table1 LEFT JOIN table2 ON table1.field1 compopr table2.field2
说明:table1, table2参数用于指定要将记录组合的表的名称。
field1, field2参数指定被联接的字段的名称。且这些字段必须有相同的数据类型及包含相同类型的数据,但它们不需要有相同的
名称。
compopr参数指定关系比较运算符:"=", "<", ">", "<=", ">=" 或 "<>"。
如果在INNER JOIN操作中要联接包含Memo 数据类型或 OLE Object 数据类型数据的字段,将会发生错误。
mysql> SELECT * FROM a LEFT JOIN b on a.a_id>b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | NULL | NULL |
| 2 | a20050112 | 1 | 2006032401 |
| 3 | a20050113 | 1 | 2006032401 |
| 3 | a20050113 | 2 | 2006032402 |
| 4 | a20050114 | 1 | 2006032401 |
| 4 | a20050114 | 2 | 2006032402 |
| 4 | a20050114 | 3 | 2006032403 |
| 5 | a20050115 | 1 | 2006032401 |
| 5 | a20050115 | 2 | 2006032402 |
| 5 | a20050115 | 3 | 2006032403 |
| 5 | a20050115 | 4 | 2006032404 |
+------+-----------+------+------------+
11 rows in set (0.00 sec)
mysql> show create table EMPLOYEE \G;
*************************** 1. row ***************************
Table: EMPLOYEE
Create Table: CREATE TABLE `EMPLOYEE` (
`FIRST_NAME` char(20) NOT NULL,
`LAST_NAME` char(20) DEFAULT NULL,
`AGE` int(11) DEFAULT NULL,
`SEX` char(1) DEFAULT NULL,
`INCOME` float DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1
1 row in set (0.00 sec)
(1)创建并插入数据,结果如下:
CREATE TABLE `a` (
`a_id` int(11)
AUTO_INCREMENT PRIMARY KEY,
`a_id_code` char(25) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
CREATE TABLE `b` (
`b_id` int(11)
AUTO_INCREMENT PRIMARY KEY,
`b_id_code` char(25) DEFAULT NULL
) ENGINE=InnoDB DEFAULT CHARSET=latin1;
INSERT INTO a
VALUES ( 1, 'a20050111' ) , ( 2, 'a20050112' ) , ( 3, 'a20050113' ) , ( 4, 'a20050114' ) , ( 5, 'a20050115' ) ;
INSERT INTO b
VALUES ( 1, '2006032401' ) , ( 2, '2006032402' ) , ( 3, '2006032403' ) , ( 4, '2006032404' ) , ( 8, '2006032408' ) ;
结果如下:
mysql> select * from a;
| a_id | a_id_code |
+------+-----------+
| 1 | a20050111 |
| 2 | a20050112 |
| 3 | a20050113 |
| 4 | a20050114 |
| 5 | a20050115 |
+------+-----------+
5 rows in set (0.00 sec)
mysql> select * from b;
+------+------------+
| b_id | b_id_code |
+------+------------+
| 1 | 2006032401 |
| 2 | 2006032402 |
| 3 | 2006032403 |
| 4 | 2006032404 |
| 8 | 2006032408 |
+------+------------+
5 rows in set (0.01 sec)
(2)left join(左联接)
mysql> SELECT * FROM a LEFT JOIN b on a.a_id=b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | 1 | 2006032401 |
| 2 | a20050112 | 2 | 2006032402 |
| 3 | a20050113 | 3 | 2006032403 |
| 4 | a20050114 | 4 | 2006032404 |
| 5 | a20050115 | NULL | NULL |
+------+-----------+------+------------+
5 rows in set (0.00 sec)
结果说明:
left join是以A表的记录为基础的,A可以看成左表,B可以看成右表,left join是以左表为准的.
换句话说,左表(A)的记录将会全部表示出来,而右表(B)只会显示符合搜索条件的记录(例子中为: A.aID = B.bID).
B表记录不足的地方均为NULL.
(3)right join(右联接)
mysql> SELECT * FROM a right JOIN b on a.a_id=b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | 1 | 2006032401 |
| 2 | a20050112 | 2 | 2006032402 |
| 3 | a20050113 | 3 | 2006032403 |
| 4 | a20050114 | 4 | 2006032404 |
| NULL | NULL | 8 | 2006032408 |
+------+-----------+------+——————+
结果说明:
仔细观察一下,就会发现,和left join的结果刚好相反,这次是以右表(B)为基础的,A表不足的地方用NULL填充.
(4) inner join(相等联接或内联接)
mysql> SELECT * FROM a inner JOIN b on a.a_id=b.b_id;
等价于
mysql> SELECT * FROM a, b WHERE a.a_id=b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | 1 | 2006032401 |
| 2 | a20050112 | 2 | 2006032402 |
| 3 | a20050113 | 3 | 2006032403 |
| 4 | a20050114 | 4 | 2006032404 |
+------+-----------+------+------------+
4 rows in set (0.00 sec)
结果说明:
很明显,这里只显示出了 A.aID = B.bID的记录.这说明inner join并不以谁为基础,它只显示符合条件的记录.
LEFT JOIN操作用于在任何的 FROM 子句中, 组合来源表的记录。使用 LEFT JOIN 运算来创建一个左边外部联接。左边外部联接将包含了从第一个(左边)开始的两个表中的全部记录,即 使在第二个(右边)表中并没有相符值的记录。
(5)
语法:FROM table1 LEFT JOIN table2 ON table1.field1 compopr table2.field2
说明:table1, table2参数用于指定要将记录组合的表的名称。
field1, field2参数指定被联接的字段的名称。且这些字段必须有相同的数据类型及包含相同类型的数据,但它们不需要有相同的
名称。
compopr参数指定关系比较运算符:"=", "<", ">", "<=", ">=" 或 "<>"。
如果在INNER JOIN操作中要联接包含Memo 数据类型或 OLE Object 数据类型数据的字段,将会发生错误。
mysql> SELECT * FROM a LEFT JOIN b on a.a_id>b.b_id;
+------+-----------+------+------------+
| a_id | a_id_code | b_id | b_id_code |
+------+-----------+------+------------+
| 1 | a20050111 | NULL | NULL |
| 2 | a20050112 | 1 | 2006032401 |
| 3 | a20050113 | 1 | 2006032401 |
| 3 | a20050113 | 2 | 2006032402 |
| 4 | a20050114 | 1 | 2006032401 |
| 4 | a20050114 | 2 | 2006032402 |
| 4 | a20050114 | 3 | 2006032403 |
| 5 | a20050115 | 1 | 2006032401 |
| 5 | a20050115 | 2 | 2006032402 |
| 5 | a20050115 | 3 | 2006032403 |
| 5 | a20050115 | 4 | 2006032404 |
+------+-----------+------+------------+
11 rows in set (0.00 sec)
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