如何从两个表（其中每个表的一列包含相似的值）中获取值？

Question

I have two tables the queries are given below

db.execSQL("CREATE TABLE IF NOT EXISTS allquestions (num_id INTEGER PRIMARY KEY NOT NULL, questions TEXT NOT NULL,catogery TEXT NOT NULL,age INT NOT NULL)" );
db.execSQL("CREATE TABLE IF NOT EXISTS answers (num_id INTEGER PRIMARY KEY NOT NULL,questionid INT NOT NULL,answer TEXT NOT NULL)" );

Here the num_id in allqestions contains the question number and the answers are there in the table(answers) column answer.In the answer column the questionid contains the question number for the answer.So what I want is how to write the query to get questions from allquestion and its answers from table answer.

Answer 1

我认为您需要它。

select questions,answers from questions que,answers ans where que.num_id=ans.questionid;

Answer 2

You have to apply the join on both tables.

       SELECT 
      allquestions.`num_id`,allquestions.`questions`,answers.questionid,
       answers.answer from allquestions  JOIN answers ON allquestions.`num_id`
         = answers.questionid 

Answer 3

Try the following. I hope you are looking for the same.
Give a row_number for each group, then use a CASE statement.

Query

select t1.questionid,a.questions,
max(case when t1.row_number=1 then t1.answer else 0 end) as answer1,
max(case when t1.row_number=2 then t1.answer else 0 end) as answer2,
max(case when t1.row_number=3 then t1.answer else 0 end) as answer3
from allquestions a
join (
SELECT @row_number:=CASE WHEN @questionid=questionid 
THEN @row_number+1 
ELSE 1 
END AS row_number,
@questionid:=questionid AS questionid,answer
FROM answers, 
(SELECT @row_number:=0,@questionid:='') AS t
ORDER BY questionid
)t1
on a.num_id=t1.questionid
group by t1.questionid;

Fiddle demo