MySQL updates columns from left to right

Assuming you have a MySQL table with two int columns x and y and try to update with this SQL:

UPDATE t SET x = 0, y = x;

you will find both x and y become 0. However, if you use this SQL:

UPDATE t SET y = x, x = 0;

you will find y has the old value of x and x becomes 0.

See 12.2.10. UPDATE Syntax for details.

On most other RDBs, both SQLs behave in the same manner as the latter.