我不知道这个问题是否符合我的要求,但是:
我在表格中有一组问题,这些问题将按特定顺序询问客户,有时我们需要插入新问题,还需要将问题向下或向上移动。
我做了一个名为 position 的字段,还有一些按钮可以增加和减少它的位置,所以我可以使用 SELECT ... ORDER BY
但它不是很好,因为有时会出现两个或更多问题相同的位置编号,MySQL 选择它们的顺序。
那么使它完美运行的正确方法是什么?
注意:我不能使用索引来做到这一点。对一些人来说这是显而易见的,但对其他人来说,不是......
最佳答案
如果我对您的理解是正确的,您需要一种方法来在插入新问题、更改现有问题的位置或删除问题时正确管理 position
列中的值序列。
假设您有以下问题表的 DDL:
CREATE TABLE `questions` (
`id` int(11) UNSIGNED NOT NULL AUTO_INCREMENT,
`question` VARCHAR(256) DEFAULT NULL,
`position` INT(11) DEFAULT NULL,
PRIMARY KEY (`id`)
);
和这样的初始数据集
+----+------------+----------+
| id | question | position |
+----+------------+----------+
| 1 | Question 1 | 1 |
| 2 | Question 2 | 2 |
| 3 | Question 3 | 3 |
+----+------------+----------+
为了得到有序的问题列表你做的很明显
SELECT *
FROM questions
ORDER BY position;
要在问题列表的末尾插入一个新问题,您可以
INSERT INTO questions (question, position)
SELECT 'New Question', COALESCE(MAX(position), 0) + 1
FROM questions;
结果将是:
+----+--------------+----------+
| id | question | position |
+----+--------------+----------+
| 1 | Question 1 | 1 |
| 2 | Question 2 | 2 |
| 3 | Question 3 | 3 |
| 4 | New Question | 4 |
+----+--------------+----------+
要将新问题插入到列表中的特定位置(假设是位置 3),您可以使用两个查询来完成:
UPDATE questions
SET position = position + 1
WHERE position >= 3;
INSERT INTO questions (question, position)
VALUES ('Another Question', 3);
现在你有
+----+------------------+----------+
| id | question | position |
+----+------------------+----------+
| 1 | Question 1 | 1 |
| 2 | Question 2 | 2 |
| 5 | Another Question | 3 |
| 3 | Question 3 | 4 |
| 4 | New Question | 5 |
+----+------------------+----------+
交换两个问题的位置(例如,id 为 2 和 5 的问题)
UPDATE questions AS q1 INNER JOIN
questions AS q2 ON q1.id = 2 AND q2.id = 5
SET q1.position = q2.position,
q2.position = q1.position
让我们看看我们得到了什么
+----+------------------+----------+
| id | question | position |
+----+------------------+----------+
| 1 | Question 1 | 1 |
| 5 | Another Question | 2 |
| 2 | Question 2 | 3 |
| 3 | Question 3 | 4 |
| 4 | New Question | 5 |
+----+------------------+----------+
这正是当用户点击您的向上和向下按钮并提供正确的问题 ID 时您所做的。
现在,如果您想在删除问题时保持职位顺序没有间隙,您可以这样做。
要从列表末尾删除,您可以使用简单删除
DELETE FROM questions WHERE id=4;
结果
+----+------------------+----------+
| id | question | position |
+----+------------------+----------+
| 1 | Question 1 | 1 |
| 5 | Another Question | 2 |
| 2 | Question 2 | 3 |
| 3 | Question 3 | 4 |
+----+------------------+----------+
删除列表中间(或开头)的问题需要更多工作。假设我们要删除 id=5 的问题
-- Get the current position of question with id=5
SELECT position FROM questions WHERE id=5;
-- Position is 2
-- Now delete the question
DELETE FROM questions WHERE id=5;
-- And update position values
UPDATE questions
SET position = position - 1
WHERE position > 2;
终于有了
+----+--------------+----------+
| id | question | position |
+----+--------------+----------+
| 1 | Question 1 | 1 |
| 2 | Question 2 | 2 |
| 3 | Question 3 | 3 |
+----+--------------+----------+
更新:为了让我们的生活更轻松,我们可以将其全部包装在存储过程中
DELIMITER $$
CREATE PROCEDURE add_question (q VARCHAR(256), p INT)
BEGIN
IF p IS NULL OR p = 0 THEN
INSERT INTO questions (question, position)
SELECT q, COALESCE(MAX(position), 0) + 1
FROM questions;
ELSE
UPDATE questions
SET position = position + 1
WHERE position >= p;
INSERT INTO questions (question, position)
VALUES (q, p);
END IF;
END$$
DELIMITER ;
DELIMITER $$
CREATE PROCEDURE swap_questions (q1 INT, q2 INT)
BEGIN
UPDATE questions AS qs1 INNER JOIN
questions AS qs2 ON qs1.id = q1 AND qs2.id = q2
SET qs1.position = qs2.position,
qs2.position = qs1.position;
END$$
DELIMITER ;
DELIMITER $$
CREATE PROCEDURE delete_question (q INT)
BEGIN
SELECT position INTO @cur_pos FROM questions WHERE id=q;
SELECT MAX(position) INTO @max FROM questions;
DELETE FROM questions WHERE id=q;
IF @cur_pos <> @max THEN
UPDATE questions
SET position = position - 1
WHERE position > @cur_pos;
END IF;
END$$
DELIMITER ;
并像这样使用它们:
-- Add a question to the end of the list
CALL add_question('How are you today?', 0);
CALL add_question('How are you today?', NULL);
-- Add a question at a specific position
CALL add_question('How do you do today?', 3);
-- Swap questions' positions
CALL swap_questions(1, 7);
-- Delete a question
CALL delete_question(2);
关于mysql - 我们如何控制表中字段的动态排序?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/14927407/