Is it possible to extract data from a SQLite database with null values?

Hi there - I am new to databases and am running into some trouble with SQLite.

I have a SQLite database where I can retrieve data from variables with no null values successfully, but variables with empty entries throw an error in SqlDbConnector/fetchRows. I have dbprefs set with NullStringRead and NullNumberRead to 'null'; however, I get the error:

Error using matlab.depfun.internal.database.SqlDbConnector/fetchRows Unexpected NULL; (zero-based) column index: 0; details: initVecVarVecFromSqldbTypes().

% Isolate the walk of patient "McKay" 's first walk over the gaitmat. It
% should have two passes.
ID_i = num2str(ID{1})

% id is retrieved correctly
id = fetch(db, ['SELECT id FROM FootFall WHERE Gait_id = ' ID_i])

% variables with blank rows are retrieved incorrectly
PassNo = fetch(db, ['SELECT PassNo FROM FootFall WHERE Gait_ID = ' ID_i])
Blob1 = fetch(db, ['SELECT Blob1 FROM FootFall WHERE Gait_id = ' ID_i])

Is this a limitation in SQLite, or should I have dbprefs set differently? Any help appreciated.

Answer 1

[Prasad Mendu]

This could be because of a bug in the SQLite JDBC driver:

https://bitbucket.org/xerial/sqlite-jdbc/issue/81/the-method-getobject-in-rs-class-processes

MATLAB uses the wasNull() method to determine whether a null value was returned (such that we can replace it with the 'NullStringRead' value). wasNull() unexpectedly throws an exception here.

[J. Lucas McKay]

Thanks Prasad - that sounds like it explains it.

Answer 2

[Iliaz Miah]

Use Built-in function ifnull() in your query. example(For SQLite db) I would like to get 0 in return of a query if actual data is null.

MatLab code:

table2array(fetch(conn,'SELECT ifnull(column2,0) From Table1 WHERE ID=12346')
This will return 0 if column2 is empty/null for the selected row.

[Samuel]

Saved my day. :thumbs up:

[Yulin Shi]

Quite Useful!

Answer 3

[Tim Buschman]

The workaround I found was to use mksqlite package: http://mksqlite.sourceforge.net/index.html . Had no problems reading a SQLite database with NULLs.

Answer 4

[Brian]

Did you ever figure out a workaround with this? I am using 2016a and have the same issue. Is this fixed in a newer matlab?

[J. Lucas McKay]

Hey Brian - unfortunately my workaround was to avoid SQLite. No idea whether it has been fixed in an update.