mysql在一行中显示多行

Question

I have a table tbl_usi in mysql with records as below:

present_date    usi_value    deal_count    
----------------------------------------------------------
2015-10-13      b1              c1           
2015-10-12      b2              c2             
2015-10-11      b3              c3

I want to write a query that will do this using present_date field to select the present date and the date before it and display them together:

present_date    usi_value    deal_count    previous_date   previous_usi_value   previous_deal_count       
----------------------------------------------------------
2015-10-13      b1              c1         2015-10-12      b2                   c2         
2015-10-12      b2              c2         2015-10-11      b3                   c3          
2015-10-11      b3              c3         2015-10-10      b4                   c4

How do I achieve this. Thanks

Answer 1

Select everything from your table, then join it to itself, making sure the 2 joined tables are given different names so you can distinguish them (I used 'a' and 'b' here). The join offsets the dates by 1 day. Then you can select the fields you want from the joined table.

select 
  a.present_date, 
  a.usi_value, 
  a.deal_count, 
  b.present_date as previous_present_date, 
  b.usi_value as previous_usi_value, 
  b.deal_count as previous_deal_count 
from
  tbl_usi as a 
  left join tbl_usi as b
    on b.present_date = a.present_date - interval 1 day;

If you didn't already have one before, you will now want an index for the present_date column too BTW.

Answer 2

Alternative, which works when there are date gaps.

select
  a.present_date, 
  a.usi_value, 
  a.deal_count, 
  b.present_date as previous_present_date, 
  b.usi_value as previous_usi_value, 
  b.deal_count as previous_deal_count 
from
  tbl_usi as a 
  join tbl_usi as b
where
  b.present_date = (select max(present_date) from tbl_usi where present_date < a.present_date);

As with previous solution the same table is joined twice, but this time the previous row is found by way of a subquery.