If you use MySQL, note this:
mysql> create database test;
Query OK, 1 row affected (0.01 sec)mysql> use test;
Database changed
mysql> create table foo (
-> x int,
-> y int
-> );
Query OK, 0 rows affected (0.00 sec)mysql> insert into foo values (1, 2);
Query OK, 1 row affected (0.00 sec)mysql> select * from foo;
+------+------+
| x | y |
+------+------+
| 1 | 2 |
+------+------+
1 row in set (0.00 sec)mysql> update foo set x=5,y=10,x=x+y;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1 Changed: 1 Warnings: 0mysql> select * from foo;
+------+------+
| x | y |
+------+------+
| 15 | 10 |
+------+------+
1 row in set (0.00 sec)mysql>
I bolded the part that bothers me. No warnings. Double-updating the same value in the same row in the same statement throws no warnings (never mind errors!). I haven't checked the ANSI standard to see if this is mentioned, but it sure is worth noting.
oh my.