如何创建一个SQL查询，其中包含一个表中的多行作为列

Question

我有2个表，学生表和联系人表。 每个学生最多只能有3位联系人。

Students table:
ID, Name
123, John
456, Jane

Contacts table:
ID, CO, Name, Relation
123, C1, Jill, Mother
123, C2, Doug, Father
123, C3, Larry, Brother
456, C1, Luis, Father
456, C2, Jill, Mother

我希望sql输出看起来像这样：

ID, Name, C1, C1name, C1rel, C2, C2name, C2rel, C3, C3name, C3rel
123, John, C1, Jill, Mother, C2, Doug, Father, C3, Larry, Brother
456, Jane, C1, Luis, Father, C2, Jill, Mother

这是为了导出到上载到第三方供应商。 谢谢。

Answer 1

您可以使用case语句或子查询。

SELECT ID, Name,

  (SELECT CO FROM Contacts c
   WHERE s.ID = c.ID
   AND c.CO = "C1") END AS C1,

  (SELECT Name FROM Contacts c
   WHERE s.ID = c.ID
   AND c.CO = "C1") END AS C1name,

  (SELECT Relation FROM Contacts c
   WHERE s.ID = c.ID
   AND c.CO = "C1") END AS C1rel
FROM Students s

等等