Hola, me encuentro realizando una APP para el control de ingresos de artículos por compra y al detallarlos con sus respectivos valores, necesito que se muestre el total de este movimiento, considerando que realicé una tabla de registro de compra y otra con los articulos recibidos, enlazadas por el número de ingreso.
He buscado orientación en muchos lugares, pero no encuentro la solución.
Agradecido por vuestra ayuda.
You may prefer writing in English for a faster response 
"Hello, I am making an APP for the control of income of items for purchase and detailing them with their respective values, I need to show the total of this movement, considering that I made a table of purchase record and another with the items received, linked by the income number.
I have looked for guidance in many places, but I can’t find the solution.
Thank you for your help."
If you established a relation, you probably used reference type columns.
You can then use dereferenced expressions, as described here:
Dereference expressions - AppSheet Help
Build list dereferences - AppSheet Help
From your description, I assume you could try, in the INCOME table, to add a virtual column with this expression:
SUM([Related PURCHASEs][value])
For reference:
“Column ‘SUMA TOTAL’ in Table ‘ENTRADAS_Schema’ of Column Type ‘Number’ has an invalid expression in the Formula field.‘=SUM([NUMERO][SUB TOTAL])’. Column ‘NUMERO’ is used in a SELECT or list dereference expression and should be a List/EnumList of Refs” This message appears to me.
Can you share a screenshot of your table columns from your app editor?
Thank you very much, friend, I solved it by creating the syntax this way, guided by what AppSheet suggested.
SUM(SELECT(ENTRADAS[CANTIDAD],[CODIGO]=[_THISROW].[CODIGO]))[_RowNumber]
