update column with multiple values

Dear list,

sorry, I already posted this, but it did not seem to have been accepted by
the mailing list. So here's my second try:

I need to update all rows of a column ('parent') based on a subquery that
returns me the first four digits of another column ('id'):

UPDATE admin SET parent=(SELECT SUBSTR(id,1,4) FROM admin);


After executing, I get the following error-message:

"more than one row returned by a subquery used as an expressionmore than one
row returned by a subquery used as an expression"


I am not quite sure about that, but maybe I need to construct a
function/loop to fulfill this task?

thanks for help.

Stefan 

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