如何从一个表中选择一列，从另一个表中选择另一列

【问题标题】：How to select a column from a table and another column from another table如何从一个表中选择一列，从另一个表中选择另一列
【发布时间】：2019-05-02 11:34:28
【问题描述】：

试图创建一个数据库，其中包含一个问题表和一个问题答案表。

如何选择一个问题（来自 poll_question）和该问题的答案（来自 poll_answer）？

CREATE TABLE poll_question(Id_question int primary key not null, question varchar(60));

CREATE TABLE poll_answer(Id_answer int primary key not null, answer varchar(100));

INSERT INTO poll_question(Id_questao, questao)
VALUES(1,"What kind of paper is allowed in tommorows exam?");

INSERT INTO poll_answer(Id_answer,answer)
VALUES(1,"A4 squared sheet");

INSERT INTO poll_answer(Id_answer,answer)
VALUES(2,"A4 lined sheet");

【问题讨论】：

提示：INNER JOIN.
您需要在“poll_answer”中使用/添加“Id_question”作为参考，作为外键建立表之间的JOIN。

标签： mysql sql mysql-workbench

【解决方案1】：

您的poll_answer 表不完整。它需要另一列来指示每个答案属于哪个问题，例如

CREATE TABLE poll_answer(Id_answer int primary key not null, Id_question int, answer varchar(100));
INSERT INTO poll_answer(Id_answer,Id_question,answer)
VALUES(1,1,"A4 squared sheet"),
(2,1,"A4 lined sheet");

然后您可以使用JOIN 找到给定问题的答案：

SELECT q.question, a.answer
FROM poll_question q
JOIN poll_answer a ON a.Id_question = q.Id_question

输出：

question                                            answer
What kind of paper is allowed in tommorows exam?    A4 squared sheet
What kind of paper is allowed in tommorows exam?    A4 lined sheet

Demo on dbfiddle

【讨论】：