From: Warren Young
Date: June 30 2008 6:09am
Subject: Re: Query::update
List-Archive: http://lists.mysql.com/plusplus/7713
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On Jun 29, 2008, at 11:23 PM, Soul Boy wrote:

> you need to access the database twice

No, actually, you don't.  The second parameter to the sql_create_x  
sets the first N members of the SSQLS as key fields, which are used in  
building the UPDATE statement.

Thus, if I have a table with a single unique key field (call it "id"),  
then I can update the row where id=42 like this:

     sql_create_X(foo, 1, 0,
         mysqlpp::sql_int, id,
         other, fields,
         go, here)
     foo new_row(5000, init, other, parameters);
     query.update(foo(42), new_row);
     query.execute();

This builds a query of the form: "UPDATE foo SET (id=5000,...) WHERE  
(id=42)"  This works because we said the first field of foo is the  
unique key column, so we got a 1-parameter ctor taking  
mysqlpp::sql_int.  We passed a temporary foo to update(), with only  
this first field set.