insert and fetch strings and matrices to/from MySQL with Matlab - mysql

Question

I need to store data in a database. I have installed and configured a MySQL database (and an SQLite database) in Matlab. However I cannot store and retrieve anything other than scalar numeric values.
% create an empty database called test_data base with MySQL workbench.
% connect to it in Matlab
conn=database('test_database','root','XXXXXX','Vendor','MySQL');
% create a table to store values
create_test_table=['CREATE TABLE test_table (testID NUMERIC PRIMARY KEY, test_string VARCHAR(255), test_vector BLOB, test_scalar NUMERIC)'];
curs=exec(conn,create_test_table)
Result is good so far (curs.Message is an empty string)
% create a new record
datainsert(conn,'test_table',{'testID','test_string','test_vector','test_scalar'},{1,'string1',[1,2],1})
% try to read out the new record
sqlquery='SELECT * FROM test_table8';
data_to_view=fetch(conn,sqlquery)
Result is bad:
data_to_view =
1 NaN NaN 1
From the documentation for "fetch" I would expect:
data_to_view =
1×4 table
testID test_string test_vector test_scalar
_____________ ___________ ______________ ________
1 'string1' 1x2 double 1
Until I learn how to read blobs I'd even be willing to accept:
data_to_view =
1×4 table
testID test_string test_vector test_scalar
_____________ ___________ ______________ ________
1 'string1' NaN 1
I get the same thing with an sqlite database. How can I store and then read out strings and blobs and why isn't the data returned in table format?

Answer 1

Matlab does not document that the default options for SQLite and MySQL database retrieval are to attempt to return everything as a numeric array. One only needs this line:
setdbprefs('DataReturnFormat','cellarray')
or
setdbprefs('DataReturnFormat','table')
in order to get results with differing datatypes. However! now my result is:
data_to_view =
1×4 cell array
{[2]} {'string1'} {11×1 int8} {[1]}
If instead I input:
datainsert(conn,'test_table',{'testID','test_string','test_vector','test_scalar'},{1,'string1',typecast([1,2],'int8'),1})
Then I get:
data_to_view =
1×4 cell array
{[2]} {'string1'} {16×1 int8} {[1]}
which I can convert like so:
typecast(data_to_view{3},'double')
ans =
1 2
Unfortunately this does not work for SQLite. I get:
data_to_view =
1×4 cell array
{[2]} {'string1'} {' �? #'} {[1]}
and I can't convert the third part correctly:
typecast(unicode2native(data_to_view{1,3}),'double')
ans =
0.0001 2.0000
So I still need to learn how to read an SQLite blob in Matlab but that is a different question.