MySQL JSON_OBJECTAGG

You must execute the code again to save those values in the database.

CREATE TABLE Posts (
  PostId INT AUTO_INCREMENT KEY,
  PostTitle VARCHAR(255),
  PostSummary VARCHAR(255)
);

INSERT INTO Posts(PostTitle, PostSummary) VALUES ('Title 1', 'Summary 1');
INSERT INTO Posts(PostTitle, PostSummary) VALUES ('Title 2', 'Summary 2');
INSERT INTO Posts(PostTitle, PostSummary) VALUES ('Title 3', 'Summary 3');

CREATE TABLE Tags (
  TagId INT AUTO_INCREMENT KEY,
  TagName VARCHAR(255),
  TagSlug VARCHAR(255)
);

INSERT INTO Tags(TagName, TagSlug) VALUES ('tag name 1', 'tag-name-1');
INSERT INTO Tags(TagName, TagSlug) VALUES ('tag name 2', 'tag-name-2');
INSERT INTO Tags(TagName, TagSlug) VALUES ('tag name 3', 'tag-name-3');

CREATE TABLE TagsMap (
  PostId INT,
  TagId INT
);

INSERT INTO TagsMap(PostId, TagId) VALUES (1,1);
INSERT INTO TagsMap(PostId, TagId) VALUES (1,2);
INSERT INTO TagsMap(PostId, TagId) VALUES (1,3);
INSERT INTO TagsMap(PostId, TagId) VALUES (2,2);

WITH
  p AS (SELECT * FROM Posts ORDER BY PostId DESC LIMIT 20)
(SELECT p.PostId, p.PostTitle, p.PostSummary,
  JSON_OBJECTAGG(t.TagSlug, t.TagName) AS TagsObject
FROM p
INNER JOIN TagsMap tm ON p.PostId = tm.TagId
INNER JOIN Tags t ON tm.TagId = t.TagId
GROUP BY p.PostId)
UNION ALL
(SELECT p.PostId, p.PostTitle, p.PostSummary,
   JSON_OBJECT()
FROM p
LEFT OUTER JOIN TagsMap tm ON p.PostId = tm.PostId
WHERE tm.TagId IS NULL
GROUP BY p.PostId)
ORDER BY PostId DESC;

PostId	PostTitle	PostSummary	TagsObject
3	Title 3	Summary 3	{"tag-name-3": "tag name 3"}
3	Title 3	Summary 3	{}
2	Title 2	Summary 2	{"tag-name-2": "tag name 2"}
1	Title 1	Summary 1	{"tag-name-1": "tag name 1"}