在MySQL的所有记录组中选择一组记录中的特定记录

Question

我有一张包含许多不同产品规格记录的表，因为它们有不同的颜色，产品在表中可能会出现多次。为了在屏幕上显示产品，我需要用黄色来选择它们的列表，但是如果黄色不存在，我需要蓝色，否则我不想要这个产品。

简化产品例如：

 
+----+--------+ 
| ID | NAME | 
+----+--------+ 
| 1 | Prod A | 
| 2 | Prod B | 
| 3 | Prod C | 
| 4 | Prod D | 
+----+--------+ 

简单求参数表：

 
+----+------------+--------+ 
| ID | ID_PRODUCT | COLOR | 
+----+------------+--------+ 
| 1 |   1 | BLUE | 
| 2 |   1 | YELLOW | 
| 3 |   2 | RED | 
| 4 |   2 | PINK | 
| 5 |   3 | BLUE | 
| 6 |   3 | GRAY | 
| 7 |   4 | YELLOW | 
+----+------------+--------+ 

预期结果：

 
+----+------------+--------+ 
| ID | ID_PRODUCT | COLOR | 
+----+------------+--------+ 
| 2 |   1 | YELLOW | 
| 5 |   3 | BLUE | 
| 7 |   4 | YELLOW | 
+----+------------+--------+ 

原始SQL这个例子：

 
CREATE TABLE `colors` (
    `ID` int(11) NOT NULL, 
    `ID_PRODUCT` int(11) DEFAULT NULL, 
    `COLOR` varchar(16) DEFAULT NULL, 
    PRIMARY KEY (`ID`) 
) ENGINE=InnoDB DEFAULT CHARSET=latin1; 

CREATE TABLE `products` (
    `ID` int(11) NOT NULL, 
    `NAME` varchar(16) DEFAULT NULL, 
    PRIMARY KEY (`ID`) 
) ENGINE=InnoDB DEFAULT CHARSET=latin1; 

INSERT INTO `colors` VALUES (1,1,'BLUE'),(2,1,'YELLOW'),(3,2,'RED'),(4,2,'PINK'),(5,3,'BLUE'),(6,3,'GRAY'),(7,4,'YELLOW'); 

INSERT INTO `products` VALUES (1,'Prod A'),(2,'Prod B'),(3,'Prod C'),(4,'Prod D'); 

Answer 1

下面是使用conditional aggregation一个选项：

select id_product, 
     max(case when color = 'YELLOW' then id 
       when color = 'BLUE' then id 
      end), 
     max(case when color = 'YELLOW' then color 
       when color = 'BLUE' then color 
      end) 
from colors 
where color in ('YELLOW','BLUE') 
group by id_product 

• SQL Fiddle Demo

Answer 2

这里有一个方法：

select c.* 
from colors c 
where c.color = 'YELLOW' 
union all 
select c.* 
from colors c 
where c.color = 'BLUE' and 
     not exists (select 1 
        from colors c2 
        where c2.id_product = c.id_product and c2.color = 'YELLOW' 
       ); 

Answer 3

如果它总是将是蓝色和黄色比较那么我d只需使用max()函数，例如

select id, id_product, max(color) from colors 
where color in ('BLUE','YELLOW') 
group by id_product;