在一个语句中包含两个限制子句? [英] Two Limit Clauses in one statement?

问题描述

我正在尝试构建一个产品评论页面，用户可以在其中评论任何提交的评论，例如亚马逊.我希望页面显示 3 条用户评论以及对每条评论的 2 回复(如果存在).

I am trying to build a product review page where users can comment any submitted reviews like Amazon. I want the page to display 3 user reviews as well as 2 responses to each of the reviews if they exist.

这是表格

CREATE TABLE product_review
    (`ID` int, `username` varchar(21), `review_title` varchar(30), `review_or_reply` int)
;

INSERT INTO product_review
    (`ID`, `username`, `review_title`, `review_or_reply`)
VALUES
    (1, 'Tom', 'Rip-off', 0),
    (2, 'Peter', 'Rip-off', 1),
    (3, 'May', 'Rip-off', 1),
    (4, 'June', 'Rip-off', 1),
    (5, 'Tommy', 'Worth the Price', 0),
    (6, 'Sammy', 'Worth the Price', 1),
    (7, 'Sam', 'Worth the Price',1),
    (8, 'Bryan','Worth the Price',1),
    (9, 'Sally', 'Average Product', 0)
;

review_or_reply 字段实际上是一个 Yes 或 No 字段，其中 0 表示这是一条评论，1 是其他用户对该评论的评论.

The review_or_reply field is effectively a Yes or No field, where 0 means it's a review and 1 is the review's comments by other users.

是否有一个 select 语句可以限制 3 条评论并提出两条评论?例如:

Is there a single select statement that can limit 3 reviews and bring up two of their comments? For example:

Select `username`,`review_title`,`reply` from product_review where review_or_reply ='0' Limit 3
Select `username`,`review_title`,`reply` from product_review where review_or_reply = '1' and title = 'Rip-off' Limit 2
Select `username`,`review_title`,`reply` from product_review where review_or_reply = '1' and title = 'Worth the Price'  Limit 2
Select `username`,`review_title`,`reply` from product_review where review_or_reply = '1' and title = 'Average Product'  Limit 2

我希望输出是这样的:

username   review_title     review_or_reply
Tom          Rip-off         0
Peter        Rip-off         1
May          Rip-off         1
Tommy        Worth the Price 0
Sammy        Worth the Price 1
Sam          Worth the Price 1
Sally        Average Product 0

推荐答案

这将返回 3 个 review_titles，然后拉出两个回复

this will return 3 review_titles and then pull out two responses to that

SELECT 
    pr.*, 
    IF( @A = t.review_title, 
        IF(@B = 3, @B := 1, @B := @B +1)
        , @B
    ) AS group_col, 
    @A := t.review_title 
FROM (
    SELECT 
        id, 
        username,
        review_title 
    FROM product_review 
    WHERE reply ='0' LIMIT 3
) t
JOIN product_review pr ON pr.review_title=t.review_title
CROSS JOIN (SELECT @A := "", @B := 1) AS temp
GROUP BY group_col, review_title
ORDER BY id;

<小时>

如果数据库中有多个回复是 0，那么这个查询将检查它.(因为您确实在其他查询中指定回复必须为 1).

if there are more than one reply that is 0 in the database like so then this query will check for that. (since you did specify in the other queries that the reply had to be 1).

INSERT INTO product_review
    (`ID`, `username`, `review_title`, `reply`)
VALUES
    (1, 'Tom', 'Rip-off', 0),
    (2, 'Peter', 'Rip-off', 1),
    (3, 'May', 'Rip-off', 0),
    (4, 'June', 'Rip-off', 1),
    (5, 'Tommy', 'Worth the Price', 0),
    (6, 'Sammy', 'Worth the Price', 1),
    (7, 'Sam', 'Worth the Price',1),
    (8, 'Bryan','Worth the Price',1),
    (9, 'Sally', 'Average Product', 0),
    (10, 'Timothy', 'Rip-off', 1)

注意，在 id 3 处有 0 的回复，而 id 10 的回复为 1.此查询将正确跳过回复 = 0.

notice that at id 3 there is a reply of 0 with id 10 a reply of 1. this query will correctly skip the reply = 0.

SELECT 
    pr.*, 
    IF( @A = t.review_title, 
        IF(pr.reply = 0, 1, 
            IF(@B = 3, @B := 1, @B := @B +1)
        ), @B
    ) AS group_col, 
    @A := t.review_title 
FROM (
    SELECT 
        DISTINCT
        id, 
        username,
        review_title 
    FROM product_review 
    WHERE reply ='0' 
    GROUP BY review_title
    LIMIT 3
) t
JOIN product_review pr ON pr.review_title=t.review_title
CROSS JOIN (SELECT @A := "", @B := 1) AS temp
GROUP BY group_col, review_title
ORDER BY id;

演示

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