Set the target table in the Table to Update area, and the target field in the Column to Update area.
In the Value area, enter rnd(1) - this will generate a decimal fraction (i.e. values between 0 and 1).
This can be modified to create more specific ranges:
rnd(1) * 100 - creates values between 0 and 100
rnd(1) * 10 +10 - creates values between 10 and 20
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