VALUES(col_name)

In an INSERT ... ON DUPLICATE KEY UPDATE statement, you can use the VALUES(col_name) function in the UPDATE clause to refer to column values from the INSERT portion of the statement. In other words, VALUES(col_name) in the UPDATE clause refers to the value of col_name that would be inserted, had no duplicate-key conflict occurred. This function is especially useful in multiple-row inserts. The VALUES() function is meaningful only in the ON DUPLICATE KEY UPDATE clause of INSERT statements and returns NULL otherwise. See Section 15.2.7.2, “INSERT ... ON DUPLICATE KEY UPDATE Statement”.

INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);

INSERT INTO table (a,b,c) VALUES (1,2,3),(4,5,6)
ON DUPLICATE KEY UPDATE c=VALUES(a)+VALUES(b);