Filter table based on other filtered table

You really can do what AlexisOlson advises, but if you'd rather use List.Contains, that's legit as well, like if you don't want the values from Table A (but my first thought was Inner Join as well):

Table.SelectRows(TableB, each List.Contains(List.Buffer(List.Distinct(TableA[FieldX])), [FieldX]))

--Nate

Thanks for this code.

I copied it very carefully, substituting the real names for the dummy ones, but failed dismally. 

More by happenstance than skill, I arrived at 

Table.SelectRows(dbo_TableB, each List.Contains(List.Buffer(List.Distinct(TableA[FieldX])), [FieldX]))

I can see what's happening, and why this is doing what I need.

dbo_TableB is referring to the underlying table in the database.  That's good.

TableA is teferring to the filtered TableA which exists after the steps in the M code.   That's also good.

Reading the M code as functional programming, and referring to the docs for each method, makes sense of it all.  I've attached the real code for information.

let
    Source = Sql.Database("redwinghealthserver.database.windows.net", "SMIPHC"),
    dbo_PrimaryPatientObservation = Source{[Schema="dbo",Item="PrimaryPatientObservation"]}[Data],
    JustThePatients = Table.SelectRows(dbo_PrimaryPatientObservation, each List.Contains(List.Buffer(List.Distinct(PrimaryPatient[NHSNumber])), [NHSNumber]))
in
    JustThePatients

You can use that as the table to join with.

= Table.NestedJoin(
    Source, {"FieldX"},
    Table.SelectRows(TableA, each [P] = P), {"FieldX"},
    "TableA", JoinKind.Inner
  )