MySQL的多行相同的表

Question

我有一个MySQL表保存配置数据的行，即：

id item value1 value2 
2 class ship bow 
3 class car  tires 
5 reg  ship level1 
7 reg  ship level2 
9 reg  car  level5 

我试图创建一个选择所有行项目=“阶级”的查询，并返回数据集匹配初始查询+所有行，其中item ='reg'和value1 = value1（来自初始结果）。

所以在这种情况下，结果集应该是这样的：

class ship bow reg ship level1 
class ship bow reg ship level2 
class car tires reg car level5 

我有点沮丧，并希望这是有道理的。感谢任何指向正确方向的指针！

Answer 1

您必须自行加入表格。假设你的表被称为configs（因为你没有告诉我们它叫什么），这样的事情应该工作：

select t1.item, t1.value1, t1.value2, t2.item, t2.value1, t2.value2 
    from configs as t1 
    inner join configs as t2 
    on t2.value1 = t1.value1 and t2.item = 'reg' 
    where t1.item = 'class'; 

如果你需要的所有行已不匹配的结果返回以及reg行，将inner join更改为left outer join。如果您希望此查询性能良好，请确保您在item和value1上有索引。

这里是概念的快速证据显示上述作品查询：

mysql> create table configs (
    -> id int unsigned primary key auto_increment, 
    -> item varchar(32) not null, 
    -> value1 varchar(32) not null, 
    -> value2 varchar(32) not null, 
    -> index (item), 
    -> index (value1) 
    ->) engine=innodb charset=utf8; 
Query OK, 0 rows affected (0.01 sec) 

mysql> insert into configs (id, item, value1, value2) values 
    -> (2, 'class', 'ship', 'bow'), 
    -> (3, 'class', 'car', 'tires'), 
    -> (5, 'reg', 'ship', 'level1'), 
    -> (7, 'reg', 'ship', 'level2'), 
    -> (9, 'reg', 'car', 'level5'); 
Query OK, 5 rows affected (0.00 sec) 
Records: 5 Duplicates: 0 Warnings: 0 

mysql> select * from configs; 
+----+-------+--------+--------+ 
| id | item | value1 | value2 | 
+----+-------+--------+--------+ 
| 2 | class | ship | bow | 
| 3 | class | car | tires | 
| 5 | reg | ship | level1 | 
| 7 | reg | ship | level2 | 
| 9 | reg | car | level5 | 
+----+-------+--------+--------+ 
5 rows in set (0.00 sec) 

mysql> select t1.item, t1.value1, t1.value2, t2.item, t2.value1, t2.value2 
    -> from configs as t1 
    -> inner join configs as t2 
    -> on t2.value1 = t1.value1 and t2.item = 'reg' 
    -> where t1.item = 'class'; 
+-------+--------+--------+------+--------+--------+ 
| item | value1 | value2 | item | value1 | value2 | 
+-------+--------+--------+------+--------+--------+ 
| class | ship | bow | reg | ship | level1 | 
| class | ship | bow | reg | ship | level2 | 
| class | car | tires | reg | car | level5 | 
+-------+--------+--------+------+--------+--------+ 
3 rows in set (0.00 sec) 

Answer 2

SELECT 
    t1.*, 
    t2.* 
FROM 
    table t1 
    LEFT JOIN table t2 ON t2.item = 'reg' AND t2.value1 = t1.value1 
WHERE 
    t1.item = 'class'