Simple example illustrating how in a MySQL UPDATE statement the order of column value pairs in the SET clause matters.

MySQL UPDATE set order matters

/*
 * Beware that the orer of set operations in MySQL are evaluated from left to right for updating a single table,
   and in no guarenteed order when updating multiple tables at once through a join.
   http://dev.mysql.com/doc/refman/5.7/en/update.html
 */

DROP TABLE IF EXISTS test;

CREATE TABLE test (
    num1 int,
    num2 int
);

INSERT INTO test (num1, num2) 
VALUES (1, 2);

UPDATE test
SET num2 = num1 + 2, num1 = num1 + 1;

SELECT num1, num2 
FROM test;
/* 2, 3 */

DROP TABLE IF EXISTS test;

CREATE TABLE test (
    num1 int,
    num2 int
);

INSERT INTO test (num1, num2) 
VALUES (1, 2);

UPDATE test
SET num1 = num1 + 1, num2 = num1 + 2;

SELECT num1, num2 
FROM test;
/* 2, 4 */