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如何在MySQL中使用SELF JOIN？

2024-04-24 09:41:03 438

要使用SELFJOIN，让我们创建一个表。创建表的查询如下-

mysql> create table SelfJoinDemo
   -> (
   -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> CountryName varchar(20),
   -> CountryRank int,
   -> `Year` varchar(10)
   -> );

现在，您可以使用insert命令在表中插入一些记录。查询如下-

mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('US',1,'2016');
mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('UK',5,'2013');
mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('France',45,'2010');
mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('Turkey',3,'2000');
mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('Japan',78,'1995');
mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('Romania',110,'2007');
mysql> insert into SelfJoinDemo(CountryName,CountryRank,`Year`) values('UK',3,'2000');

使用select语句显示表中的所有记录。查询如下-

mysql> select *from SelfJoinDemo;

以下是输出-

+----+-------------+-------------+------+
| Id | CountryName | CountryRank | Year |
+----+-------------+-------------+------+
|  1 | US          |           1 | 2016 |
|  2 | UK          |           5 | 2013 |
|  3 | France      |          45 | 2010 |
|  4 | Turkey      |           3 | 2000 |
|  5 | Japan       |          78 | 1995 |
|  6 | Romania     |         110 | 2007 |
|  7 | UK          |           3 | 2000 |
+----+-------------+-------------+------+
7 rows in set (0.00 sec)

这是SELFJOIN的查询-

mysql> SELECT DISTINCT t1.CountryName, t2.Year
   -> FROM SelfJoinDemo AS t1,
   -> SelfJoinDemo AS t2
   -> WHERE t1.Year=t2.Year
   -> and t1.CountryName='US';

以下是输出-

+-------------+------+
| CountryName | Year |
+-------------+------+
| US          | 2016 |
+-------------+------+
1 row in set (0.00 sec

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如何在MySQL中使用SELF JOIN？

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