Using phpMyAdmin: can one alter a table's schema by inserting new columns, as one would using spreadsheet software? Like when one creates a new table.

If you click the Structure tab you'll be able to affect the columns or add in new ones.
The next screen will allow you to adjust the column properties of those selected.
Look below the table structure display
The next screen will allow you to enter the properties for each of the new columns for the table.

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