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so 1 form submit would be set Column B = "Blah" where ID = 1 -- but leave the rest of that records unchanged.
UPDATE mytable SET B = "Blah" WHERE ID = 1;
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another example might be on form submit set Column E = "Test" where ID = 19 -- but leave the rest of that records unchanged.
UPDATE mytable SET E = "Test" WHERE ID = 19;
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ie set Column E = "Test" and Column A = "Day" where ID = 19 -- but leave the rest of that records unchanged.
UPDATE mytable SET E = "Test" , A = "Day" WHERE ID = 19;
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What is confusing me is the part about leaving columns with no value from the form submit, unchanged.
Why? If you put all of the existing values for the row into the form, and edit one or more values, then write all of those values back to the row, then only those changed values are changed. All the unchanged values are overwitten with the same value.
It's up to you when you build the form what happens to populate the form in the first place, and what happens with the values from that form when it is submitted.
Good luck,
Barry.