new example: many-to-many

See #29

Author: lovasoa

examples/modeling a many to many relationship with a form/README.md

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+# A simple blog with topics and posts
+
+Users can create topics and posts. A topic can have many posts and a post can have many topics.
+
+See 
+ - [`write.sql`](./write.sql) for the form definition,
+ - [`write_submit.sql`](./write_submit.sql) for the database insertion code.
+
+![](./screenshots/home.png)
+![](./screenshots/topics.png)
+![](./screenshots/topic.png)
+![](./screenshots/post.png)
+![](./screenshots/write.png)

examples/modeling a many to many relationship with a form/index.sql

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+SELECT * FROM sqlpage_shell LIMIT 1;
+
+SELECT 'list' as component,
+    COALESCE(
+        (SELECT name FROM topic WHERE id = $topic),
+        'Recent blog posts'
+    ) as title;
+
+SELECT post.title as title,
+    'post.sql?id=' || post.id as link,
+    'Published on ' || created_at as description,
+    CASE
+        WHEN created_at > date('now', '-2 days') THEN 'red'
+        ELSE NULL
+    END as color,
+    topic.icon as icon,
+    created_at > date('now', '-2 days') as active
+FROM post
+    LEFT JOIN topic ON topic.id = post.main_topic_id
+WHERE $topic IS NULL
+    OR topic.id = $topic
+    OR EXISTS (
+        SELECT 1
+        FROM topic_post
+        WHERE topic_post.topic_id = $topic
+            AND topic_post.post_id = post.id
+    )
+ORDER BY created_at DESC;
+
+SELECT 'text' AS component;
+SELECT 'No blog post yet. ' AS contents WHERE NOT EXISTS (SELECT 1 FROM post);
+SELECT 'Write a post !' AS contents, 'write.sql' AS link;

examples/modeling a many to many relationship with a form/post.sql

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+SELECT *
+FROM sqlpage_shell
+LIMIT 1;
+
+SELECT 'text' AS component;
+
+SELECT content || '
+
+---
+
+Published on: _' || created_at || '_ in category [' || topic.name || '](/?topic=' || topic.id || ')' || '.
+    
+Other associated categories: ' || (
+        SELECT group_concat('[' || name || '](/?topic=' || topic_id || ')', ', ')
+        FROM topic_post
+            INNER JOIN topic ON topic.id = topic_post.topic_id
+        WHERE post_id = $id
+    ) || '.' AS contents_md
+FROM post
+    INNER JOIN topic ON topic.id = post.main_topic_id
+WHERE post.id = $id;

examples/modeling a many to many relationship with a form/screenshots/home.png

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+-- A simple blog with topics and posts
+-- Users can create topics and posts. A topic can have many posts and a post can have many topics.
+
+-- The first step is to create the tables in the database. We will use the following SQL queries:
+CREATE TABLE topic (
+    id INTEGER PRIMARY KEY,
+    name TEXT NOT NULL,
+    icon TEXT NOT NULL,
+    UNIQUE (name)
+);
+
+CREATE TABLE post (
+    id INTEGER PRIMARY KEY,
+    title TEXT NOT NULL,
+    content TEXT NOT NULL, -- The contents will be stored in markdown
+    created_at TIMESTAMP NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    main_topic_id INTEGER REFERENCES topic(id)
+);
+
+CREATE TABLE topic_post (
+    topic_id INTEGER NOT NULL REFERENCES topic(id),
+    post_id INTEGER NOT NULL REFERENCES post(id),
+    PRIMARY KEY (topic_id, post_id)
+) WITHOUT ROWID;
+
+-- A view of the topics with the number of posts and most recent post
+CREATE VIEW topic_with_stats AS
+SELECT topic.id,
+    topic.name,
+    topic.icon,
+    COALESCE(count(topic_post.post_id), 0) as nb_posts,
+    max(post.created_at) as last_post
+FROM topic
+    LEFT JOIN topic_post ON topic_post.topic_id = topic.id
+    LEFT JOIN post ON post.id = topic_post.post_id
+GROUP BY topic.id
+ORDER BY nb_posts DESC;