string to date - hive SQL

问题

I am running queries in a hive environment.

I have a column which has a timestamp but is set up a string in the tables. I tried the following : all of them return Null

SELECT 
,To_date(activitydate)

Cast:

,cast(activitydate as timestamp)

This is the how the data is set up in the table:

Appreciate any inputs on how I can convert this :

05/12/2017 00:00:00     



SELECT 
 cust_id
,to_date(activitydate)  activity_date
,type type_of_contact


FROM repl_task

WHERE to_date(activitydate) BETWEEN '2014-01-01' AND '2017-01-01' ;

I am running out of memory if I run this :

SELECT 
 cust_id
,activitydate
,SUBSTR(activitydate,4,2) AS DT
,SUBSTR(activitydate,0,2) AS MONTH 
,SUBSTR(activitydate,7,4) AS YEAR
,type 


FROM task 

WHERE activitydate >='01/01/2016 00:00:00'

回答1:

unix_timestamp function converts given format to unix timestamp and from_unixtime function converts from unix timestamp to given format:

hive> select from_unixtime(unix_timestamp('01/01/2016 00:00:00','MM/dd/yyyy HH:m:ss'),'yyyy-MM-dd');
OK
2016-01-01
Time taken: 0.118 seconds, Fetched: 1 row(s)

回答2:

Can you try using to_date() with the date portion of your timestamp in ISO format:

SELECT 
    cust_id,
    TO_DATE(CONCAT(SUBSTR(activitydate, 7, 4), '-',
                   SUBSTR(activitydate, 0, 2), '-',
                   SUBSTR(activitydate, 4, 2))) activity_date
    type type_of_contact
FROM repl_task
WHERE
    TO_DATE(CONCAT(SUBSTR(activitydate, 7, 4), '-',
                   SUBSTR(activitydate, 0, 2), '-',
                   SUBSTR(activitydate, 4, 2)))
        BETWEEN '2014-01-01' AND '2017-01-01';

If this runs, but is not very performant, then you should consider storing your timestamps in ISO format. Storing dates as text, or as text in a non standard format, carries a penalty with most databases.

来源：https://stackoverflow.com/questions/45377124/string-to-date-hive-sql

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