In Dapper, the order of columns matters when performing 1-to-many mappings using the Query method with multi-mapping. Here's what you need to know:

Id Column Position:

1. • The split-on column (usually the Id) must appear first in the column sequence for each entity

   • Dapper splits rows when it sees a duplicate Id in this column.

     Proper Sequence Example:
     
     sql
     SELECT 
         u.Id, u.Name,  -- User columns (must start with Id)
         p.Id, p.Title, p.UserId  -- Post columns (must start with Id)
     FROM Users u
     JOIN Posts p ON u.Id = p.UserId