efficient update queries

[admin]

Does UPDATE automatically checks to see if the data has changed before actually writing anything into the database (MySQL in question here)

For example, which is more efficient:

function update($a, $b, ..., $bar) {
mysql_query("UPDATE Foo SET A = '$a', B = '$b', ... WHERE Bar = $bar");
}


or

function update($a, $b, ...) {
mysql_query("SELECT A, B, ... WHERE Bar = $bar");

...

if(A different) {
mysql_query("UPDATE Foo SET A = '$a' WHERE Bar = $bar");
}
if(B different) {
mysql_query("UPDATE Foo SET B = '$b' WHERE Bar = $bar");
}
if(...) {
...
}
}


if most of the data in the row changes, then the first function should be more efficient, however, if only one field changes, is the second function more efficient?