Postgres - 行到列

Question

我有什么

查询：

SELECT u.firstname,u.lastname, u.institution as department, u.department as company, c.shortname,  
to_char(to_timestamp(p.timecompleted)::date,'YYYY-MM-DD') AS completed
FROM mdl_course_completions AS p
JOIN mdl_course AS c ON p.course = c.id
JOIN mdl_user AS u ON p.userid = u.id
WHERE c.enablecompletion = 1 AND u.firstname is NOT NULL AND p.timecompleted is NOT NULL
ORDER BY u.firstname

结果：

firstname   lastname   department   company   course    completed
u1          u1         x            x          c1        date
u1          u1         x            x          c2        date

我需要什么

我需要能够将课程传输到列。导致类似这样的事情：

firstname   lastname   department   company   c1    c2
u1          u1         x            x         date  date
u2          u2         x            x         date  date   

我尝试过使用交叉表，但我对 SQL 的熟练程度不够。有人可以帮忙吗？

EDIT：课程数量在数百个，所以它需要是动态的。

（另外：英语不是我的母语，如有不明之处请见谅）。

Answer 1

这不是您要求的，但可能对您有用。最后一列是一个数组，其结构为：{'course_name: course_completed', ...}。

SELECT u.firstname,u.lastname, u.institution as department, u.department as company
    array_agg(c.shortname || ': ' || to_char(to_timestamp(p.timecompleted)::date,'YYYY-MM-DD')) AS courses_completed
FROM mdl_course_completions AS p
JOIN mdl_course AS c ON p.course = c.id
JOIN mdl_user AS u ON p.userid = u.id
WHERE c.enablecompletion = 1 AND u.firstname is NOT NULL AND p.timecompleted is NOT NULL
GROUP BY u.firstname,u.lastname, u.institution, u.department
ORDER BY u.firstname

输出将是：

firstname   lastname   department   company   courses_completed
u1          u1         x            x         {"c1: date", "c2: date"}
u2          u2         x            x         {"c1: date", "c2: date"}

其他选项是使用交叉表功能（如您所说）：Postgresql crosstab documentation。您必须安装 tablefunc 模块才能使用交叉表。阅读文档，如果您认为它对您有用，我可以帮助您编写查询（但我需要了解表结构和一些示例数据）。

Answer 2

您可以使用条件聚合，例如使用FILTER 子句：

SELECT
    firstname, lastname, department, company,
    MAX(completed) FILTER (WHERE course = 'c1') as c1,
    MAX(completed) FILTER (WHERE course = 'c2') as c2
FROM 
    -- <joins>
GROUP BY firstname, lastname, department, company

您需要按属于同一组的列进行分组。然后，您可以在要旋转的列上执行您喜欢的任何聚合函数。使用附加的FILTER 子句，您可以过滤所有应识别为聚合的记录。