title: MySQL decimal field 'Data truncated for column x at row 1' issue
body: <p>I have a mysql table with a decimal(16,2) field. Seems like the addition operation with another decimal(16,2) field string can cause the <code>Data truncated for column x at row 1</code> issue, which raises exception in my django project.</p>
  
  <p>I'm aware of multiplication or division operation of that field can cause this issue bacause the result is probably not fit in decimal(16,2) definition, but does the addition and subtraction operation the same?</p>
  
  <p>My MySQL server version is <code>5.5.37-0ubuntu0.14.04.1</code>. You can reproduce this issue from bellow:</p>
  
  <pre><code>mysql&gt; drop database test;
  Query OK, 1 row affected (0.10 sec)
  
  mysql&gt; create database test;
  Query OK, 1 row affected (0.00 sec)
  
  mysql&gt; use test;
  Database changed
  mysql&gt; create table t(price decimal(16,2));
  Query OK, 0 rows affected (0.16 sec)
  
  mysql&gt; insert into t values('2004.74');
  Query OK, 1 row affected (0.03 sec)
  
  mysql&gt; select * from t;
  +---------+
  | price   |
  +---------+
  | 2004.74 |
  +---------+
  1 row in set (0.00 sec)
  
  mysql&gt; update t set price = price + '0.09';
  Query OK, 1 row affected (0.05 sec)
  Rows matched: 1  Changed: 1  Warnings: 0
  
  mysql&gt; update t set price = price + '0.09';
  Query OK, 1 row affected, 1 warning (0.03 sec)
  Rows matched: 1  Changed: 1  Warnings: 1
  
  mysql&gt; show warnings;
  +-------+------+--------------------------------------------+
  | Level | Code | Message                                    |
  +-------+------+--------------------------------------------+
  | Note  | 1265 | Data truncated for column 'price' at row 1 |
  +-------+------+--------------------------------------------+
  1 row in set (0.00 sec)
  
  mysql&gt; select * from t;
  +---------+
  | price   |
  +---------+
  | 2004.92 |
  +---------+
  1 row in set (0.00 sec)
  </code></pre>