SQL Server 2017

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CREATE TABLE Tbl(PK INT, Char1 VARCHAR(100), Char2 VARCHAR(100), Measure INT)

INSERT INTO Tbl VALUES(1,'A','ch_k',1),(1,'B','ch_m',2),(1,'B','ch_n',3),
(2,'A','ch_k',1),(2,'A','ch_j',3),(2,'B','ch_n',2),(2,'C','ch_l',5);

SELECT PK,MIN(Mode_A) Mode_A, MAX(Mode_B) Mode_B,
MIN(CASE WHEN Acnt > 1 THEN Mode_A ELSE Mode_B END) Mode_AB,
MIN(sum_A)sum_A,MIN(sum_B)sum_B,MIN(Total)Total
FROM (
  SELECT DISTINCT 
    PK, 
    CASE WHEN Char1 = 'A' THEN Char2 Else Null END 'Mode_A',
    CASE WHEN Char1 = 'B' THEN Char2 Else Null END 'Mode_B',
    COUNT(CASE WHEN Char1 = 'A' THEN Char2 Else Null END) OVER(PARTITION BY PK ORDER BY PK) Acnt,
    COUNT(CASE WHEN Char1 = 'B' THEN Char2 Else Null END) OVER(PARTITION BY PK ORDER BY PK) Bcnt,
    SUM(CASE WHEN Char1 = 'A' THEN Measure ELSE 0 END) OVER(PARTITION BY PK ORDER BY PK) AS sum_A,
    SUM(CASE WHEN Char1 = 'B' THEN Measure ELSE 0 END) OVER(PARTITION BY PK ORDER BY PK)AS sum_B,
    SUM(Measure) OVER(PARTITION BY PK ORDER BY PK) AS Total
  FROM Tbl
) X
GROUP BY PK;

PK	Mode_A	Mode_B	Mode_AB	sum_A	sum_B	Total
1	ch_k	ch_n	ch_m	1	5	6
2	ch_j	ch_n	ch_j	4	2	11

Warning: Null value is eliminated by an aggregate or other SET operation.