我将如何消除包含concat的更新查询中的重复项

Question

Here is the query I have to call each time a new item is added to the database :

UPDATE `cas_phocagallery`,`cas_phocagallery_categories`
   SET cas_phocagallery.description=concat(
       cas_phocagallery_categories.title,'<br />',cas_phocagallery.description)
 WHERE cas_phocagallery.catid = cas_phocagallery_categories.id;

It works fine except that it produces a duplicate each time I run the query. I tried to add distinct but I get an error and I am not sure this is the right thing to do in this case.

Thank you for your cooperation, I have been looking at all kinds of ways all day with no success.

Answer 1

When you have a unique key, it should produce an error when inserting a duplicate value and is expected behaviour. I think what you should be looking at is on duplicate key update where you can specify what to do if a duplicate is found.

Answer 2

Every time you run the query, you add the title to the current description . If you run it three times in a row, you will get

title    <-- added by 3rd
title    <-- added by 2nd
title    <-- added by 1st
description (original)

Is that what you mean? That is exactly what the query is designed to do. Maybe you need another column to keep the "base_description", so that each time, "description" gets to be "title" + br + "base_description"

---

EDIT

You can try something like this, which would work 95% of the time, until the title changes in which case you will get the historical titles built up. It also doesn't work when the title contains the % symbol and may work funny if it contains the _ character.

UPDATE cas_phocagallery, cas_phocagallery_categories
   SET cas_phocagallery.description=concat(
       cas_phocagallery_categories.title,'<br />',cas_phocagallery.description)
 WHERE cas_phocagallery.catid = cas_phocagallery_categories.id
   AND NOT cas_phocagallery.description
           LIKE concat(cas_phocagallery_categories.title, '%')

Can't really comment on Joomla or how it does things or why this would be necessary (to merge title into description). What you should really do is in some SELECT statement in some module, show the result of the CONCAT instead of persisting (pushing/mangling) it into description.

   SELECT cas_phocagallery.*, concat(cas_phocagallery_categories.title,
                                     '<br />',
                                     cas_phocagallery.description) as FullDescription
     FROM cas_phocagallery
LEFT JOIN cas_phocagallery_categories
       ON cas_phocagallery.catid = cas_phocagallery_categories.id