MySQL lag column DATEDIFF -

suppose have following data:

customer_id contact_id  comm_creation_day days_since_last_contact 10000       28036173295 2014-12-21        0 10000       28365672745 2015-01-29        datediff('2015-01-29','2014-12-21') 10000       28576719155 2015-02-26        datediff('2015-02-26','2014-01-29') 38409       28432217395 2015-02-07        0 38409       28565986955 2015-02-25        datediff('2015-02-25,'2015-02-07') 

i'm trying days_since_last_contact column, there's no lead or lag function in mysql. how do in single select statement?

i tried following:

select customer_id, comm_id , comm_creation_day, previous_comm_creation_day  ( select c.* , @prev previous_comm_creation_day , @prev := comm_creation_day contacts c, (select @prev:=null) vars order customer_id, c.comm_creation_day, c.comm_id ) sq order customer_id, comm_creation_day 

but gave me:

customer_id contact_id  comm_creation_day last_contact_date 10000       28036173295 2014-12-21        null 10000       28365672745 2015-01-29        2014-12-21 10000       28576719155 2015-02-26        2015-01-29 38409       28432217395 2015-02-07        2015-02-26 (this incorrect!) 38409       28565986955 2015-02-25        2015-02-07 

where null doesn't show last column in cases.

any ideas?

yes in mysql need use user defined variables such calculations as

select customer_id, contact_id, contact_date, days_since_last_contact  from(   select   customer_id,   contact_id,   contact_date,   @diff:= if(@prev_customer = customer_id,datediff(contact_date,@prev_date),0) days_since_last_contact,   @prev_customer:=customer_id,   @prev_date:= contact_date   customer c,(select @prev_customer:=0,@prev_date:=null)x   order customer_id,contact_date )x 

http://sqlfiddle.com/#!9/6cb9f/2