Question

This situation involves 3 tables.
items
videos
instructions

Each Item has a column ‘entitytype’ and a handle ‘entityid’ which is the corresponding id of a row in either table videos or table instructions. Unfortunately the name of the id’s in these tables is urlid and textid respectively.

I would like to select an Item, and Join the data from either a row in videos or a row in instructions onto the selection based on what value ‘entitytype’ holds

This code is psuedo, I am new to MySQL and trying to find a means to convey my intentions

Here’s my intuition thus far:

SELECT items.*, 
    IF(entitytype='video', 'videos', 'instructions') as detail_table,
    IF(entitytype='video', 'urlid', 'textid') as detail_id
FROM items
    JOIN (detail_table) ON (items.entityid=detail_table.detail_id)
WHERE itemid=something

Answer 1

Just join against both tables, and select the value you need using the IF.

SELECT items.yourcolumns,
    IF(items.entitytype='video', urlid, textid) as result_id
FROM items
    LEFT OUTER JOIN videos ON items.entityid=videos.urlid
    LEFT OUTER JOIN instructions ON items.entityid=instructions.textid
WHERE itemid=something

Your relational model is broken if a column’s reference points to another table based on another columns value.

Better: have two extra tables: items_to_video and items_to_instructions which make a link where needed

Edit: after two minutes of lying in bed and not thinking about it, I thought about it. Offcourse.

 SELECT items.yourcolumns,
     urlid as result_id
 FROM items
     JOIN videos ON items.entityid=videos.urlid
 WHERE itemid=something AND items.entitytype='video'
UNION ALL
 SELECT items.yourcolumns,
     textid as result_id
 FROM items
     JOIN instructions ON items.entityid=instructions.textid
 WHERE itemid=something AND items.entitytype<>'video'