如何使用插入語句在父表的子表中添加或插入外鍵值

Question

我想在我的子表中使用父表的主鍵。 我創建了一個外鍵來連接 QUESTION 和 ANSWER 表。 下面是我的數據庫的代碼：

用戶表

CREATE TABLE user (
  user_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, -- unique index
  username VARCHAR(255) NOT NULL, 
  password VARBINARY(255) NOT NULL,                -- password hash, binary
  firstname VARCHAR(255) NOT NULL,
  lastname VARCHAR(255) DEFAULT NULL,
  email VARCHAR(255) DEFAULT NULL,
  address VARCHAR(255) DEFAULT NULL,
  phone BIGINT DEFAULT NULL
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

CREATE TABLE question (
   question_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,  -- unique index
   questions TEXT NOT NULL,    
   question_text TEXT NOT NULL,        
   user_id INT NOT NULL,                                 -- id of the user who have asked
   CONSTRAINT user_to_question FOREIGN KEY (user_id) REFERENCES user (user_id)
   ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

 CREATE TABLE answer (
  answer_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY,  -- unique index
  answer_text TEXT NOT NULL,
  question_id INT NOT NULL,                           -- what question it answers on
  user_id INT NOT NULL,                               -- id of the user who have answered
  CONSTRAINT user_to_answer FOREIGN KEY (user_id) REFERENCES user (user_id),
  CONSTRAINT question_to_answer FOREIGN KEY (question_id) REFERENCES question (question_id)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_0900_ai_ci;

因此，在 USER 表中，我添加了一個新值：

  insert into user values(1,"krish","123456789","Krish","D","krish@gmail.com","Mumbai MH","98776");

現在在 QUESTION 表中，我想添加一個新值，並且我想使用 USER 表中的 user_id 值。 由於 user_id 是 QUESTION 表中的外鍵。 所以，到目前為止，我已經嘗試過這個查詢，但它不起作用。

 insert into question (question_id,questions,question_text)
 SELECT user_id
 FROM user
 where user_id = 1;
 values (1,"What is java","Please Explain in details");

我已經提到了這個問題和解決方案，但它不起作用。 在postgres中將數據從父表插入到子表

Answer 1

您正在嘗試的查詢類似於：

insert into question ( question_id,
                       questions,
                       question_text
                     )
SELECT user_id as question_id,
       "What is java" as questions,
       "Please Explain in details" as question_text
FROM user
where user_id = 1 ;

但據我所知，你不能只插入這個值，因為

CREATE TABLE answer (
  answer_id INT NOT NULL AUTO_INCREMENT PRIMARY KEY, 
  answer_text TEXT NOT NULL,
  question_id INT NOT NULL,                           
  user_id INT NOT NULL,  

你有user_id INT NOT NULL所以它需要一個值。

就像是：

insert into question ( question_id,
                       questions,
                       question_text,
                       user_id
                      )
SELECT user_id as question_id,
       "What is java" as questions,
       "Please Explain in details" as question_text,
       user_id
FROM user
where user_id = 1 ;

Answer 2

如果您知道從user表中選擇的user_id ，則根本不需要SELECT ，只需將其放在VALUES列表中即可。

INSERT INTO question (question_id,questions,question_text, user_id)
VALUES (1,"What is java","Please Explain in details", 1);

如果您需要使用其他列查找user_id ，您將使用SELECT 。 在這種情況下，您將固定值添加到SELECT列表中。

INSERT INTO question (question_id,questions,question_text, user_id)
SELECT 1,"What is java","Please Explain in details", user_id
FROM users
WHERE username = 'krish';

如果您在創建用戶后立即執行此操作，您還可以使用LAST_INSERT_ID() function 來獲取使用自動增量分配的user_id 。

INSERT INTO question (question_id,questions,question_text, user_id)
VALUES (1,"What is java","Please Explain in details", LAST_INSERT_ID());