MS SQL Server到Postgresql

Question

This query helps me to returns the previous month by having the actual month as parameter.

SELECT SUBSTRING(CONVERT(VARCHAR(10), DATEADD(MONTH, DATEDIFF(MONTH, 0, '01-'+'2014-05') - 1, 0), 105),4,10)

for example, here the query returns : '04-2014'

so how can i do the samething in postgresql, taking into account that postgresql doesn't use :

DATEDIFF , DATEADD functions?

Answer 1

Use to_char() and to_date() :

select to_char(to_date('2014-05' || '-01', 'YYYY-MM-DD') - interval '1 month', 'MM-YYYY')

I would encourage you to use the format "YYYY-MM" instead of "MM-YYYY", if you have a choice. Putting the year first makes it possible to order by the value.