(mysql.info.gz) Design Limitations

 
 7.1.1 MySQL Design Limitations and Tradeoffs
 --------------------------------------------
 
 When using the `MyISAM' storage engine, MySQL uses extremely fast table
 locking that allows multiple readers or a single writer. The biggest
 problem with this storage engine occurs when you have a steady stream
 of mixed updates and slow selects on a single table.  If this is a
 problem for certain tables, you can use another storage engine for
 them.  Storage engines.
 
 MySQL can work with both transactional and non-transactional tables.
 To be able to work smoothly with non-transactional tables (which can't
 roll back if something goes wrong), MySQL has the following rules (when
 not running in strict mode or if you use the `IGNORE' specifier to
 `INSERT' or `UPDATE').
 
    * All columns have default values.
 
    * If you insert an "incorrect" value in a column, such as a
      too-large numeric value into a numeric column, MySQL sets the
      column to the "best possible value" instead of giving an error.
      For numerical values, this is 0, the smallest possible value or
      the largest possible value. For strings, this is either the empty
      string or the longest possible string that can be stored in the
      column.
 
    * All calculated expressions return a value that can be used instead
      of signaling an error condition. For example, 1/0 returns `NULL'.
      (This behavior can be changed by using the
      `ERROR_FOR_DIVISION_BY_ZERO' SQL mode).
 
 If you are using non-transactional tables, you should not use MySQL to
 check column content. In general, the safest (and often fastest) way is
 to let the application ensure that it passes only legal values to the
 database.
 
 For more information about this, see  Constraints and 
 INSERT or  Server SQL mode.
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