You can press CTRL+0 to enter null into the textbox. OR, before saving the data, replace empty/zero values with null.

>I have some textboxes that display values from a table. The type of these values is double. The default value is .NULL.
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>As expected, when I create a blank record, these values are initialized in the table to .NULL.
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>When the user enters a number into the textboxes, the table values get updated appropriately, i.e. the table value becomes that number.
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>However, if a user now selects that textbox and deletes its contents, the table value does not revert to .NULL., but instead becomes the value 0.0. This is not correct.
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>What is the best way to have the table value go back to .NULL. when the user deletes the value from a textbox bound to that table field?
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>Thanks,
>Jim