I have successfully used CSV import on my datatables using the INSERT and REPLACE methods but the UPDATE method does not work - if UPDATE is selected from the dropdown menu in the dialog box then the import button is greyed out. I will attach a screengrab to illustrate this. NB This applies whether the update is affecting all the fields or only a few.

When using UPDATE you must define one or more columns to match (ie. the columns that will be the WHERE expression in the UPDATE query).
To do this, align the fields and then click on the "–" in the middle to change it to "=".
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When using UPDATE you must define one or more columns to match (ie. the columns that will be the WHERE expression in the UPDATE query).
To do this, align the fields and then click on the "–" in the middle to change it to "=".