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 (2,2);

SELECT p.PostTitle, p.PostSummary,
/* GROUP_CONCAT(t.TagSlug), GROUP_CONCAT(t.TagName) */
JSON_OBJECTAGG(t.TagSlug, t.TagName)
FROM (SELECT PostId, PostTitle, PostSummary
      FROM Posts) p 
LEFT JOIN TagsMap tm ON p.PostId = tm.TagId
LEFT JOIN Tags t ON tm.TagId = t.TagId
GROUP BY p.PostId 
ORDER BY p.PostId DESC;

ERROR 3158 (22032) at line 30: JSON documents may not contain NULL member names.