Returning a rows maximum value

I want to query the table to get back the maximum value (in this instance, the most recent date) from each row. Please see attached for sample data (fig 1) and desired output (fig. 2)

I could probably do this using the attached sql but was wondering if there was a more efficient way of doing this?

thanks
select
client_id,
max(date_field) max_date
from
(select Client_id, start_date as date_field from sample_table
union
select Client_id, end_date as date_field from sample_table
union
select Client_id, update_date as date_field from sample_table
union
select Client_id, reassign_date as date_field from sample_table)
group by
client_id

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Sample.xls
tonMachine100Asked:
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awking00Connect With a Mentor Commented:
see attached.
maxdate.txt
0
 
ajexpertConnect With a Mentor Commented:
If you know that there is no duplicate data in sample_table then use UNION ALL instead of UNION as UNION ALL will definately improve performance
select
client_id,
max(date_field) max_date
from
(select Client_id, start_date as date_field from sample_table
UNION ALL
select Client_id, end_date as date_field from sample_table
UNION ALL 
select Client_id, update_date as date_field from sample_table
UNION ALL
select Client_id, reassign_date as date_field from sample_table)
group by
client_id

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0
 
MikeOM_DBACommented:
Try GREATEST() function:

SQL> Alter Session Set Nls_Date_Format='Mm/Dd/Yyyy';
 
Session altered.
 
SQL> 
SQL> With Sample_Table As (
  2  Select 1234  Client_Id
  3  , '2/1/2008' Start_Date
  4  , '6/5/2008' End_Date
  5  , '6/4/2008' Update_Date
  6  , '6/3/2008' Reassign_Date
  7  From Dual Union
  8  Select 1235,'8/5/2006',Null,'9/5/2006',Null From Dual Union
  9  Select 1236,'9/7/2005','10/8/2008','10/15/2006',Null From Dual Union
 10  Select 1237,'1/4/2008','5/4/2008','2/3/2008','2/3/2008' From Dual Union
 11  Select 1238,'5/1/2008','6/6/2008',Null,Null From Dual)
 12  Select Client_Id
 13       , Greatest(Nvl(Start_Date,'1/1/1900')
 14                 ,Nvl(End_Date,'1/1/1900')
 15                 ,Nvl(Update_Date,'1/1/1900')
 16                 ,Nvl(Reassign_Date,'1/1/1900'))
 17    From Sample_Table
 18  /
 
 CLIENT_ID GREATEST(N
---------- ----------
      1234 6/5/2008
      1235 9/5/2006
      1236 9/7/2005
      1237 5/4/2008
      1238 6/6/2008
 
SQL> 

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0
 
MikeOM_DBAConnect With a Mentor Commented:
Ooops, her is with date format corrected:

SQL> Alter Session Set Nls_Date_Format='Mm/Dd/Yyyy';
 
Session altered.
 
SQL> With Sample_Table As (
  2  Select 1234  Client_Id
  3  , '02/01/2008' Start_Date
  4  , '06/05/2008' End_Date
  5  , '06/04/2008' Update_Date
  6  , '06/03/2008' Reassign_Date
  7  From Dual Union
  8  Select 1235,'08/05/2006',Null,'09/05/2006',Null From Dual Union
  9  Select 1236,'09/07/2005','10/08/2008','10/15/2006',Null From Dual Union
 10  Select 1237,'01/04/2008','05/04/2008','02/03/2008','02/03/2008' From Dual Union
 11  Select 1238,'05/01/2008','06/06/2008',Null,Null From Dual)
 12  Select Client_Id
 13       , Greatest(Nvl(Start_Date,'01/01/1900')
 14                 ,Nvl(End_Date,'01/01/1900')
 15                 ,Nvl(Update_Date,'01/01/1900')
 16                 ,Nvl(Reassign_Date,'01/01/1900'))
 17*   From Sample_Table
SQL> /
 
 CLIENT_ID GREATEST(N
---------- ----------
      1234 06/05/2008
      1235 09/05/2006
      1236 10/15/2006
      1237 05/04/2008
      1238 06/06/2008
 
SQL> 

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