SELECT Column1, Column2,
CASE WHEN column1 = 'datapoint1' THEN DATE_SUB(column2, INTERVAL 1 MONTH)
WHEN column1 = 'datapoint2' THEN DATE_SUB(column2, INTERVAL 2 MONTH)
END as Column3
You can also do this in MagicETL, which might be worth doing if you are not comfortable with SQL. You would use the Filter tile to split your data into two groups: datapoint1 and datapoint2. You would then use the Date Operations tile on both of those and use the subtract from date function in the tile and subtract 1 month for datapoint1 and 2 months for datapoint2. You would then use the Append tile to bring the datasets back together.
Hope this helps.
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