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I'm very new to sql/hive. At first, I loaded a txt file into hive using:

drop table if exists Tran_data;
create table Tran_data(tran_time string, 
resort string, settled double)
ROW FORMAT DELIMITED FIELDS TERMINATED BY '\t' LINES TERMINATED BY '\n';
Load data local inpath 'C:\Users\me\Documents\transaction_data.txt' into table Tran_Data;
The variable tran_time in the txt file is like this:10-APR-2014 15:01. After loading this Tran_data table, I tried to convert tran_time to a "standard" format so that I can join this table to another table using tran_time as the join key. The date format desired is 'yyyymmdd'. I searched online resources, and found this: unix_timestamp(substr(tran_time,1,11),'dd-MMM-yyyy') 
So essentially, I'm doing this: unix_timestamp('10-APR-2014','dd-MMM-yyyy'). However, the output is "NULL".
So my question is: how to convert the date format to a "standard" format, and then further convert it to 'yyyymmdd' format?
                Above function is valid only for valid date formats, but some times we get "00122013" which is in 'MMddyyyy" format , and there is no month here, but is there any way to display it as "2013-12-00" , when i tried using above function, output is giving diff value.2012-12-12
– user2883028
                Dec 5, 2019 at 23:43
My current Hive Version: Hive 0.12.0-cdh5.1.5
I converted datetime in first column to date in second column using the below hive date functions. Hope this helps!
select inp_dt, from_unixtime(unix_timestamp(substr(inp_dt,0,11),'dd-MMM-yyyy')) as todateformat from table;
  
inp_dt                  todateformat

  12-Mar-2015 07:24:55    2015-03-12 00:00:00
unix_timestamp function will convert given string date format to unix timestamp in seconds , but not like this format dd-mm-yyyy.
You need to write your own custom udf to convert a given string date to the format that you need as present Hive do not have any predefined functions. We have to_date function to convert a timestamp to date , remaining all unix_timestamp functions won't help your problem.
                Thank you for your reply. However I notice that unix_timestamp('20140501','yyyymmdd') can return something that's not null. However, unix_timestamp('01-MAY-2014','dd-mmm-yyyy') gives me a "Null". Where's the difference between the two?
– Yuning Zhang
                Aug 8, 2014 at 19:37
unix_timestamp('2014-05-01','dd-mmm-yyyy') will work, your input string should be in this format for hive yyyy-mm-dd or yyyy-mm-dd hh:mm:ss
Where as you are trying with '01-MAY-2014' hive won't understand it as a date string
        
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