多对多SQL查询，用于选择带有某些单词标记的所有图像 [英] Many to Many SQL Query for selecting all Images Tagged with certain words

问题描述

我在Postgres中有2个表:

I have 2 Tables in Postgres:

CREATE TABLE "images" (
    "id" serial NOT NULL PRIMARY KEY,
    "title" varchar(300) NOT NULL,
    "relative_url" varchar(500) NOT NULL)

和

CREATE TABLE "tags" (
    "id" serial NOT NULL PRIMARY KEY,
    "name" varchar(50) NOT NULL)

要在图像和标签之间建立多对多关系，我需要另外一张表:

To establish many to many relationship between images and tags I have another table as:

CREATE TABLE "tags_image_relations" (
    "id" serial NOT NULL PRIMARY KEY,
    "tag_id" integer NOT NULL REFERENCES "tags" ("id") DEFERRABLE INITIALLY DEFERRED,
    "image_id" integer NOT NULL REFERENCES "images" ("id") DEFERRABLE INITIALLY DEFERRED)

现在，我必须编写一个查询，例如"选择所有标记有'apple'，'microsoft'和'google'的图像的relative_url"

Now I have to write a query like "select relative_url of all images tagged with 'apple' and 'microsoft' and 'google' "

对此最优化的查询有哪些?

What can the most optimized query for this?

推荐答案

这是我编写的有效查询:

Here's the working query I wrote:

SELECT i.id, i.relative_url, count(*) as number_of_tags_matched
FROM   images i
    join tags_image_relations ti on i.id = ti.image_id
    join tags t on t.id = ti.tag_id
    where t.name in ('google','microsoft','apple')
    group by i.id having count(i.id) <= 3
    order by count(i.id)

此查询将首先显示与所有三个标签匹配的图像，然后显示与3个标签中的至少2个匹配的图像，最后显示与1个标签匹配的图像.

This query will first show the images matching all three tags, then the images matching at least 2 of the 3 tags, finally at least 1 tag.

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