Question

I have tables to organize tags..

3 tables for articles and article tags:

article_tags
---
tag_id *
tag_name

articles
---
article_id *
article_name 

individual_article_tags
----
article_id *
tag_id *

AND 3 separate tables for images and images tags

image_tags
---
tag_id *
tag_name

images
---
image_id *
image_name 

individual_image_tags
----
image_id *
tag_id *

I want the articles and images tables to reference Only 1 tag table instead of 2 tag tables like so:

tags
---
tag_id *
tag_name

articles
---
article_id *
article_name 

individual_article_tags
----
article_id *
tag_id *

images
---
image_id *
image_name 

individual_image_tags
----
image_id *
tag_id *

The problem is that the tags have different IDs and I don’t know how to merge them..

Answer 1

First, create an new tags_unified table with its own auto_increment id (which will differ from either of the other existing tables) by doing INSERT INTO ...SELECT with a UNION. The result will be a distinct set of all tags from both tables.

CREATE TABLE tags_unified (
  tags_id INT PRIMARY KEY AUTO_INCREMENT,
  tag_name VARCHAR(64)
);

/* Load up all the tags with new ids */
INSERT INTO tags_unified (tag_name)
  SELECT tag_name FROM article_tags
  UNION
  SELECT tag_name FROM image_tags

Then UPDATE all the individual_image_tags and inidividual_article_tags with a JOIN query to get the new ids.

UPDATE
  individual_image_tags
  JOIN image_tags ON individual_image_tags.tag_id = image_tags.tag_id
  /* LEFT JOIN the old tags to the new tags by tag_name */
  LEFT JOIN tags_unified ON image_tags.tag_name = tags_unified.tag_name
SET individual_image_tags.tag_id = tags_unified.tag_id
/* And update those which have a match in the new tags table */
WHERE tags_unified.tag_id IS NOT NULL

UPDATE
  individual_article_tags
  JOIN article_tags ON individual_article_tags.tag_id = article_tags.tag_id
  LEFT JOIN tags_unified ON article_tags.tag_name = tags_unified.tag_name
SET individual_article_tags.tag_id = tags_unified.tag_id
WHERE tags_unified.tag_id IS NOT NULL

Before running the UPDATE statements, re-form them as SELECT statements to verify the results.

SELECT
  individual_image_tags.*, 
  tags_unified.*
FROM
  individual_image_tags
  JOIN image_tags ON individual_image_tags.tag_id = image_tags.tag_id
  LEFT JOIN tags_unified ON image_tags.tag_name = tags_unified.tag_name
WHERE tags_unified.tag_id IS NOT NULL

SELECT
 individual_article_tags.*, 
 tags_unified.*
FROM
  individual_article_tags
  JOIN article_tags ON individual_article_tags.tag_id = article_tags.tag_id
  LEFT JOIN tags_unified ON article_tags.tag_name = tags_unified.tag_name
WHERE tags_unified.tag_id IS NOT NULL

Edit:

The LEFT JOIN and the WHERE clause should not actually be necessary, because the should be a match for every existing tag in the tags_unified table.

Update after comments:

It may be easier to create temporary tables and repopulate the original link tables from those than to drop and re-add unique constraints or composite keys. Use CREATE TEMPORARY TABLE AS SELECT.... Then delete all rows from the original table and use INSERT INTO ... SELECT to re-fill it from the temp table.

CREATE TEMPORARY TABLE temp_individual_article_tags AS
    SELECT
     individual_article_tags.article_id 
     tags_unified.tag_id
    FROM
      individual_article_tags
      JOIN article_tags ON individual_article_tags.tag_id = article_tags.tag_id
      LEFT JOIN tags_unified ON article_tags.tag_name = tags_unified.tag_name
    WHERE tags_unified.tag_id IS NOT NULL