postgres sql to extract year-month

Question

Have a table with a column like this:

   first_day_month         
   01/07/2020    
   01/07/2020    
   01/08/2020    
   01/09/2020   
  .......

Need to create a column like year-month, Tried to_char(first_day_month, 'MM/YYYY') but got an error:

Error running query: INVALID_FUNCTION_ARGUMENT: Failed to tokenize string [M] at offset [0]

Tried

concat(extract(year from first_day_month),'-',extract(month from first_day_month) ) as month,

with an error:

Error running query: SYNTAX_ERROR: line 2:1: Unexpected parameters (bigint, varchar(1), bigint) for function concat. Expected: concat(array(E), E) E, concat(E, array(E)) E, concat(array(E)) E, concat(varchar)

Also tried date_parse but didn't get it right, any idea? Thanks

Answer 1

You need to use TO_DATE first, to convert the column to a proper date. Then use TO_CHAR to format as you want:

SELECT TO_CHAR(TO_DATE(first_day_month, 'DD/MM/YYYY'), 'MM/YYYY') AS my
FROM yourTable;

Note that in this case since the text month year you want is actually just the right substring, you could also directly use RIGHT here:

SELECT RIGHT(first_day_month, 7)
FROM yourTable;

Finally, note that YYYY/MM would generally be a better format to use, as it sorts properly. So perhaps consider using this version:

SELECT TO_CHAR(TO_DATE(first_day_month, 'DD/MM/YYYY'), 'YYYY/MM') AS ym
FROM yourTable;

Answer 2

Your data doesn't seem to be of DATE type, might be string, then need to convert to DATE type first and format display style as desired pattern :

SELECT TO_CHAR(first_day_month::DATE,'MM/YYYY') AS first_day_month
  FROM t

Demo