嗨我有两张桌子
Student
--------
Id Name
1 John
2 David
3 Will
Grade
---------
Student_id Mark
1 A
2 B
2 B+
3 C
3 A
是否有可能使本机Postgresql选择得到这样的结果:
Name Array of marks
-----------------------
'John', {'A'}
'David', {'B','B+'}
'Will', {'C','A'}
但不是这样的
Name Mark
----------------
'John', 'A'
'David', 'B'
'David', 'B+'
'Will', 'C'
'Will', 'A'
使用array_agg:http://www.sqlfiddle.com/#!1/5099e/1
SELECT s.name, array_agg(g.Mark) as marks
FROM student s
LEFT JOIN Grade g ON g.Student_id = s.Id
GROUP BY s.Id
顺便说一下,如果你使用的是Postgres 9.1,你就可以将SELECT到SELECT BY的列don't need to repeat,例如您不需要在GROUP BY上重复学生姓名。您只能在主键上使用GROUP BY。如果删除学生的主键,则需要在GROUP BY上重复学生姓名。
CREATE TABLE grade
(Student_id int, Mark varchar(2));
INSERT INTO grade
(Student_id, Mark)
VALUES
(1, 'A'),
(2, 'B'),
(2, 'B+'),
(3, 'C'),
(3, 'A');
CREATE TABLE student
(Id int primary key, Name varchar(5));
INSERT INTO student
(Id, Name)
VALUES
(1, 'John'),
(2, 'David'),
(3, 'Will');
据我所知,你可以这样做:
SELECT p.p_name,
STRING_AGG(Grade.Mark, ',' ORDER BY Grade.Mark) As marks
FROM Student
LEFT JOIN Grade ON Grade.Student_id = Student.Id
GROUP BY Student.Name;
编辑
我不确定。但也许这样的话:
SELECT p.p_name,
array_to_string(ARRAY_AGG(Grade.Mark),';') As marks
FROM Student
LEFT JOIN Grade ON Grade.Student_id = Student.Id
GROUP BY Student.Name;
参考here
您可以使用以下内容:
SELECT Student.Name as Name,
(SELECT array(SELECT Mark FROM Grade WHERE Grade.Student_id = Student.Id))
AS ArrayOfMarks
FROM Student
如下所述:http://www.mkyong.com/database/convert-subquery-result-to-array/
@Michael Buen做对了。我使用array_agg得到了我需要的东西。
这里只是一个基本的查询示例,以防有人帮助:
SELECT directory, ARRAY_AGG(file_name)
FROM table
WHERE type = 'ZIP'
GROUP BY directory;
结果如下:
parent_directory | array_agg | ------------------------+----------------------------------------+
/home/postgresql/files | {zip_1.zip,zip_2.zip,zip_3.zip} |
/home/postgresql/files2 | {file1.zip,file2.zip} |
这篇文章对我帮助很大:"Group By" in SQL and Python Pandas。它基本上说尽可能只使用SQL更方便,但Python Pandas可以在过滤过程中实现额外的功能。
我希望它有所帮助