如何從兩個表（其中每個表的一列包含相似的值）中獲取值？

Question

我有兩個表，查詢如下

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)" );

在這里，所有問題中的num_id包含問題號，並且答案在表（answers）列的答案中。在答案列中，questionid包含答案的問題號。所以我想要的是如何編寫查詢以獲取問題從allquestion及其表答案中得到答案。

Answer 1

我認為您需要它。

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

Answer 2

您必須在兩個表上應用聯接。

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

Answer 3

嘗試以下方法。 希望您也一樣。
為每個組指定一個row_number，然后使用CASE語句。

詢問

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;

小提琴演示