How to add data validation unique input in column

How to add unique constraint in column so data will be unique from each column data

There is no direct way but we can do this by using formula in validity options

Goto data menu
Select validity
Click on criteria
In allow option Select custom
Write your formula

For example if we want our first column should contains unique values then we will write formula as

COUNTIF(A:A, A1) = 1

Then click on Error alert
And choose action which you want when duplicate values Entered

I hope you understood.

The solution to allow only unique data input is to use a database and give column(s) the unique constraint.