Teachers with Students in common

DThayer

Here is another way.

First create a new Table from the Modelling Tab>>New Table

Table =
ALL ( Table1[Teachers] )

Now you can use this MEASURE in your original Table (say Table1)

Measure =
VAR mytable =
    UNION ( VALUES ( Table1[Teachers] ), VALUES ( 'Table'[Teachers] ) )
VAR myrows =
    COUNTROWS ( DISTINCT ( mytable ) )
VAR M1 =
    COUNTROWS (
        FILTER (
            VALUES ( Table1[Students] ),
            CALCULATE ( COUNT ( Table1[Teachers] ), Table1[Teachers] IN mytable )
                = 2
        )
    )
VAR M2 =
    COUNT ( Table1[Students] )
VAR M3 =
    IF ( myrows = 1, M1, M2 )
RETURN
    IF ( ISBLANK ( M3 ), 0, M3 )

DThayer - I published the final version of this to the Quick Measure gallery here:

https://community.powerbi.com/t5/Quick-Measures-Gallery/In-Common/td-p/382956

It has the PBIX and a longer explanation.

Power Query solution:

Table Students:

TeacherStudent

Create 2 tables of distinct teachers with a second column of "1" to join the tables together:

Students(2):

TeacherCustom

 Students(3):

TeacherCustom

Now merge the 2 new tables, creating a list of all possible combinations of teacher relationships.  Next, merge the newly created list back to the "Students"  - twice - once on column "Teacher" and then on column "Teacher.1".  Add a column to denote if the students match, then filter out non-matches.  Group the data by "Teacher" and "Teacher.1" to get the count, then simply pivot the data around "Teacher.1".  Here is the actual code:

let
    Source = Table.NestedJoin(#"Students (3)",{"Custom"},#"Students (2)",{"Custom"},"Students (2)",JoinKind.LeftOuter),
    #"Expanded Students (2)" = Table.ExpandTableColumn(Source, "Students (2)", {"Teacher"}, {"Teacher.1"}),
    #"Removed Columns" = Table.RemoveColumns(#"Expanded Students (2)",{"Custom"}),
    #"Merged Queries" = Table.NestedJoin(#"Removed Columns",{"Teacher"},Students,{"Teacher"},"Students",JoinKind.LeftOuter),
    #"Expanded Students" = Table.ExpandTableColumn(#"Merged Queries", "Students", {"Student"}, {"Student"}),
    #"Merged Queries1" = Table.NestedJoin(#"Expanded Students",{"Teacher.1"},Students,{"Teacher"},"Students",JoinKind.LeftOuter),
    #"Expanded Students1" = Table.ExpandTableColumn(#"Merged Queries1", "Students", {"Student"}, {"Student.1"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Students1", "Match", each if [Student] = [Student.1] then 1 else 0),
    #"Filtered Rows" = Table.SelectRows(#"Added Custom", each ([Match] = 1)),
    #"Removed Columns1" = Table.RemoveColumns(#"Filtered Rows",{"Student", "Student.1"}),
    #"Grouped Rows" = Table.Group(#"Removed Columns1", {"Teacher", "Teacher.1"}, {{"Count", each Table.RowCount(_), type number}}),
    #"Pivoted Column" = Table.Pivot(#"Grouped Rows", List.Distinct(#"Grouped Rows"[Teacher.1]), "Teacher.1", "Count", List.Sum)
in
    #"Pivoted Column"