Add text to the start of a SOLR database value - sql

Question

I am using this code for my SOLR DIH:
<dataConfig>
<dataSource name="app" driver="org.postgresql.Driver" url="jdbc:postgresql://localhost:5432/wikipedia" user="wikipedia" password="secret" />
<dataSource name="data" driver="org.postgresql.Driver" url="jdbc:postgresql://localhost:5432/wikipedia" user="wikipedia" password="secret" />
<document>
<entity dataSource="app-ds" name="item" query="SELECT id, title, description, date, link, location_id, source_id, company_id from item">
<field column="id" name="id" />
<field column="title" name="title" />
<field column="description" name="description" />
<field column="date" name="date" />
<field column="link" name="link" />
<entity dataSource="app-ds" name="location" query="SELECT name, coordinate from location where location_id=${item.location_id}">
<field column="name" name="location_name" />
<field column="coordinate" name="location_coordinates" />
</entity>
<entity dataSource="app-ds" name="source" query="SELECT name from source where source_id=${item.source_id}">
<field column="name" name="source_name" />
</entity>
<entity dataSource="app-ds" name="company" query="SELECT name from company where company_id=${item.company_id}">
<field column="name" name="company_name" />
</entity>
</entity>
</document>
</dataConfig>
Since I am merging two databases I want to have a uniqueID for each entry within SOLR. In my case the best way of doing this is to have app*ID* for the first databases ID's and data*ID* for the second databases ID's.
Using my code above, how do I add the word "app" to the front of the ID that is stored in the SOLR ID field so that my database ID=123 and the Solr ID = app123
EDIT: As I guess it might be something like this (but I am not good with SQL)
query="SELECT app_(id)

Answer 1

You can try to modify the SQL Query as -
SELECT 'TABLE1' || ID AS PRIMARY_ID ........
PRIMARY_ID can be now configured as unique ID in solr.
<field column="primary_id" name="primary_id" />
Even if you are performing incremental updates using Delta imports, the SQL ID updated would still generate the Same Solr ID with the above query and would be updated. So be sure to use the above in the Delta queries as well.