MySQL选择一列DISTINCT,以及相应的其他列

Question

ID   FirstName   LastName
1      John        Doe
2      Bugs        Bunny
3      John        Johnson

我想DISTINCT从FirstName列中选择结果,但我需要相应的ID和LastName.

结果集只需显示一个John,但需要ID1和1 LastName的Doe.

Answer 1

试试这个查询

 SELECT ID, FirstName, LastName FROM table GROUP BY(FirstName)

Answer 2

该DISTINCT关键字并未真正发挥你期待它的方式.当你使用时,SELECT DISTINCT col1, col2, col3你实际上选择了所有唯一的{col1,col2,col3}元组.

Answer 3

编辑

原始答案是在MySQL 5.7.5之前编写的,由于默认更改而不再适用ONLY_FULL_GROUP_BY.同样重要的是要注意,当ONLY_FULL_GROUP_BY禁用时,使用GROUP BY没有聚合函数将产生意外结果,因为MySQL可以自由选择被分组的数据集中的任何值[原文如此].

假设firstname和lastname是唯一索引的,GROUP BY则可以使用a LEFT JOIN来对结果集进行排序.见示范

按降序检索由姓氏排序的不同名字(ZA)

SELECT t1.*
FROM table_name AS t1
LEFT JOIN table_name AS t2
ON t1.firstname = t2.firstname
AND t1.lastname < t2.lastname
WHERE t2.id IS NULL;

#Results
| id | firstname | lastname |
|----|-----------|----------|
|  2 |      Bugs |    Bunny |
|  3 |      John |  Johnson |

按升序检索由姓氏排序的不同名字(AZ)

SELECT t1.*
FROM table_name AS t1
LEFT JOIN table_name AS t2
ON t1.firstname = t2.firstname
AND t1.lastname > t2.lastname
WHERE t2.id IS NULL;

#Results
| id | firstname | lastname |
|----|-----------|----------|
|  2 |      Bugs |    Bunny |
|  1 |      John |      Doe |

然后,您可以根据需要订购结果数据.

如果名字和姓氏组合不是唯一的并且您有多行具有相同值,则可以通过在联接上包含OR条件来筛选结果集以选择特定ID.看演示.

table_name数据:

(1, 'John', 'Doe'),
(2, 'Bugs', 'Bunny'),
(3, 'John', 'Johnson'),
(4, 'John', 'Doe'),
(5, 'John', 'Johnson')

SELECT t1.*
FROM table_name AS t1
LEFT JOIN table_name AS t2
ON t1.firstname = t2.firstname
AND (t1.lastname > t2.lastname
OR (t1.firstname = t1.firstname AND t1.lastname = t2.lastname AND t1.id > t2.id))
WHERE t2.id IS NULL;

#Results
| id | firstname | lastname |
|----|-----------|----------|
|  1 |      John |      Doe |
|  2 |      Bugs |    Bunny |

警告

使用时,使用MySQL GROUP BY并不会总是产生预期的结果ORDER BY:参见:测试用例示例.

确保预期结果的最佳实现方法是使用像这样的子查询来过滤结果集范围.

table_name数据:

(1, 'John', 'Doe'),
(2, 'Bugs', 'Bunny'),
(3, 'John', 'Johnson')

询问

SELECT * FROM (
   SELECT * FROM table_name ORDER BY ID DESC
) AS t1
GROUP BY FirstName

#Results
| ID | first |    last |
|----|-------|---------|
|  2 |  Bugs |   Bunny |
|  3 |  John | Johnson |

与

SELECT * FROM table_name GROUP BY FirstName ORDER BY ID DESC

#Results
| ID | first |  last |
|----|-------|-------|
|  2 |  Bugs | Bunny |
|  1 |  John |   Doe |

Answer 4

SELECT ID,LastName 
From TABLE_NAME 
GROUP BY FirstName 
HAVING COUNT(*) >=1

Answer 5

怎么样

`SELECT 
    my_distinct_column,
    max(col1),
    max(col2),
    max(col3)
    ...
 FROM
    my_table 
 GROUP BY 
    my_distinct_column`