FIRST DAY of a MONTH in Teradata

Computing the first day of a month:

SELECT 'input date' - EXTRACT(DAY FROM 'input date') + 1; 

If you want to calculate the first day of '2012-06-06'
SELECT '2012-06-06' - EXTRACT(DAY FROM '2012-06-06') + 1; 

Result: '2012-06-01'

The code above subtracts the number of days from the input month, taking you to "day 0" or the day before the first of the month. Then it will add 1 which yield first day of the month.