c# - Unexplainable MySQL composite key issue with .NET Application -

i writing .net application interact mysql database using odbc connection. application create database schema , tables on database upon start up. however, encountered weird unexplainable case below:

first application create following table

create table if not exists `sample` ( `item_id` varchar(20) not null, `item_no` int(11) not null, `sample_col1` varchar(20) not null, `sample_col2` varchar(20) not null, primary key (`item_id`, `item_no`) ) engine=innodb; 

populate table with

insert sample set item_id='abc', item_no=1, sample_col1='', sample_col2=''; insert sample set item_id='abc', item_no=2, sample_col1='', sample_col2=''; 

then execute select query within .net application using following code:

dim query string query = "select item_id, item_no, sample_col1, sample_col2 sample"  dim conn new odbcconnection("driver={mysql odbc 5.1 driver};server=localhost; port=3306;database=test; user=dbuser;password=dbpassword;option=3;") dim cmd new odbcdataadapter(query, conn) dim dt new datatable cmd.fillschema(dt, schematype.source) cmd.fill(dt) conn.close() 

the cmd.fill(dt) line throw exception: "failed enable constraints. 1 or more rows contain values violating non-null, unique, or foreign-key constraints".

however if modify table creation query to:

create table if not exists `sample` ( `item_id` varchar(20) not null, `item_no` int(11) not null, `sample_col1` varchar(20) not null, `sample_col2` varchar(20) not null, primary key (`item_no`,`item_id`)  '--> here primary key order inverted ) engine=innodb; 

the vb.net code works perfectly. notice on second creation query inverted order of primary key creation. on first example put item_id first while on second example put item_no first.

does has clue why happening? there difference point of view of mysql database between first , second create query?

any input appreciated. thank !