How to modify the built-in 'fetch' function so that it can select columns flexibly?

Question

Simon 2022-11-9

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1847158-how-to-modify-the-built-in-fetch-function-so-that-it-can-select-columns-flexibly

评论： Simon 2022-11-10

I was trying to write a function which fetch only the columns I like from a MySQL table.

easy_fetch(conn, 'mytable', 'A', 'B') % error, incorrect sqlquery string
easy_fecth(conn, 'mytable', 'A,', 'B') % this works but clumsy
function T = easy_fetch (conn, tablename, varargin)
sqlquery = ['select '  varargin{:} ' from ' tablename]; % I don't know how to insert commas here.
T = fetch(conn, sqlquery);
end

That fuction does not create right sql query string because it doesn't have commas between A and B

wrong_string = 'select A B from mytable'.
fetch(conn, wrong_string) % error
% the correct query string should have commas between A and B
right_string = 'select A, B from mytable'
fecth(conn, right_string) % success

How would I put ',' between A and B in the easy_fetch function? Or is there better way to flexibly fetch columns? I have checked out a couple packages shared in File Exchange. None has codes I can use. Thanks for any suggestions.

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Answer 1

Steven Lord 2022-11-10

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https://ww2.mathworks.cn/matlabcentral/answers/1847158-how-to-modify-the-built-in-fetch-function-so-that-it-can-select-columns-flexibly#answer_1096343

在 MATLAB Online 中打开

s = {'Larry', 'Curly', 'and Moe'}
s = 1×3 cell array
    {'Larry'}    {'Curly'}    {'and Moe'}
s2 = strjoin(s, ', ')
s2 = 'Larry, Curly, and Moe'

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Simon 2022-11-10

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I am very thankful for your solution. It works! The modified version does what I want.

T = easy_fetch(conn, 'mytable', 'A', 'B') % works smoothly :-)
%%
function T = easy_fetch (conn, tablename, varargin)
cols = strjoin(varargin, ',');
sqlquery = ['select ' cols ' from ' tablename];
T = fetch(conn, sqlquery);
end