changing character into timestamp - postgresql

Question

I have two columns (date character varying, time character varying) in table VM. Now I want to merge these two columns and change the type as a timestamp. can anyone help me out to do this?
Or else
I want to separate data based on the year (2017, 2018). I used substring(date, '2018') command but it replies with 2017 data showing NULL instead of 2017 along with 2018 data.

Answer 1

You didn't tell us what exactly the contents of that table is, but something like this should work:
Add a new timestamp column (and please find a better name than "timestamp" or ts_column for it)
alter table the_table add ts_column timestamp;
Then you need to update the data
update the_table
set ts_column = to_timestamp(concat("date", ' ', "time"), 'yyyy-mm-dd hh24:mi:ss')
where "date" is not null;
The above assumes that the date stored in the column named "date" and the time stored in the column named "time" are formatted as ISO date and time values. If they are not, you need to adjust the parameter to the to_timestamp() function. For details on the format mask, please see the manual
Then drop the old columns:
alter table the_table drop "date", drop "time";
To extract the year from a date or timestamp column, use the extract function:
where extract(year from ts_column) = 2018
Note that the above will not be able to use an index on ts_column to speed up the query.
An alternative way of writing is:
where ts_column >= date '2018-01-01'
and ts_column < date '2019-01-01';
which would be able to use an index on ts_column