多行，一个查询

Question

假设我有下表，名为GPAs：

name | semester | GPA
Joe    Winter     3.5
Joe    Spring     4.0

如何在一个查询中返回以下内容？

name | gpaWinter | gpaSpring

Joe      3.5          4.0

Answer 1

select name, 
    max(case when semester = 'Winter' then GPA end) as gpaWinter, 
    max(case when semester = 'Spring' then GPA end) as gpaSpring
from GPAs
group by name

Answer 2

SELECT name, (SELECT GPA FROM GPAs WHERE name = t1.name AND semester = 'Winter') AS `gpaWinter`, (SELECT GPA FROM GPAs WHERE name = t1.name AND semester = 'Spring') AS `gpaSpring` FROM GPAs t1

但你真的不应该在SQL中这样做。 SQL的目的是检索数据;将其格式化以便显示是应用程序代码的工作。

Answer 3

SELECT name,GROUP_CONCAT(semester) AS semesters,GROUP_CONCAT(GPA) AS GPAs
FROM GPAs
GROUP BY name

这将返回如下行：

name |   semesters   |  GPAs
Joe    Winter,Spring   3.5,4.0

然后你可以解析学期和GPA。

您还可以执行GROUP_CONCAT(semester,'=',GPA) as GPAs之类的操作，它将返回：

name    |         GPAs
Joe       Winter=3.5,Spring=4.0

然后你可以解析GPAs行。