How to Extract Min and Max Dates from a Group By Statement in SQL

Understanding the Problem: Getting Min and Max Dates from a Group By Statement

In SQL, when performing a GROUP BY statement, it’s common to need to extract min and max values from a specific column. However, in this particular problem, we also want to get the corresponding dates for these min and max values.

The given table structure is as follows:

station datetime        calculatedpower min_power   max_power   lt_dt               lp_dt
ABBA    28AUG2018:0:0:0     100         1            100        01SEP2018:1:0:0     28AUG2018:0:0:0
ABBA    31AUG2018:12:0:0    88          1            100        01SEP2018:1:0:0     28AUG2018:0:0:0
ABBA    01SEP2018:1:0:0     1           1            100        01SEP2018:1:0:0     28AUG2018:0:0:0
ZZZZ    07SEP2018:0:0:0     900         900          3000       07SEP2018:0:0:0     21SEP2018:0:0:0
ZZZZ    09SEP2018:0:0:0     1000        900          3000       07SEP2018:0:0:0     21SEP2018:0:0:0
ZZZZ    21SEP2018:0:0:0     3000        900          3000       07SEP2018:0:0:0     21SEP2018:0:0:0

We are given the following GROUP BY statement:

proc sql;
select 
station
,datetime
,calculatedpower
,min(calculatedpower) as lt_calculatedpower
,max(calculatedpower) as lp_calculatedpower
from sumall
group by 
station
;
quit;

However, this statement does not provide us with the min and max dates (lt_dt and lp_dt) that we need. We want to extract these dates based on the minimum and maximum values of calculatedpower.

The Problem

The problem lies in identifying when each group of station has its minimum and maximum calculatedpower. In other words, for which datetime does lt_calculatedpower (the minimum calculated power) occur and which datetime does lp_calculatedpower (the maximum calculated power) occur.

Solution Overview

To solve this problem, we will use a sub-select that contains a case statement to identify the date at which the min and max occur. This sub-select is then joined to the original table.

Using Sub-Selects with Case Statements

In SQL, when using a sub-select with a case statement, we need to ensure that the sub-select returns all the columns from the outer query. In this problem, we want to join our want table with its original table (have), so we will include all columns in the sub-select.

Here’s an example of how we can structure our SQL query:

proc sql;
  create table want as
  select 
    have.*
    , min(have.x) as min_x
    , max(have.x) as max_x
    , min(at.min_at) as min_x_first_at_seq
    , min(at.max_at) as max_x_first_at_seq
  from 
    have
  left join 
  (
    select inside.level1_id, inside.level2_seq
    , case when inside.x = min(inside.x) then inside.level2_seq else . end as min_at
    , case when inside.x = max(inside.x) then inside.level2_seq else . end as max_at
    from have inside
    group by inside.level1_id
  ) at
  on 
    have.level1_id = at.level1_id and 
    have.level2_seq = at.level2_seq
  group by 
    have.level1_id
  order by 
    have.level1_id, level2_seq
;

Breaking Down the Solution

Let’s break down this solution:

We start with creating a want table that includes all columns from our original table (have).
Inside our sub-select (the left join), we use case statements to identify when each group of station has its minimum and maximum calculatedpower.
- The min function is used to find the minimum value in a group.
- We then check if the current value (inside.x) is equal to this minimum value. If it’s not, we return an empty string for that level2_seq. Otherwise, we assign its corresponding level2_seq as the minimum date (min_at).
The same logic applies for finding the maximum value.
- We use the max function to find the maximum value in a group.
- Again, we check if the current value is equal to this maximum value. If it’s not, we return an empty string for that level2_seq. Otherwise, we assign its corresponding level2_seq as the maximum date (max_at).
We then join our sub-select with the original table using a left join.
Finally, we group by station and order by both station and level2_seq to maintain the original order.

Handling Potential Issues

One potential issue arises when there are multiple rows with the same minimum or maximum value but different dates. In such cases, it’s unclear which date corresponds to that value. To handle this situation, we could consider either ignoring duplicate dates or using additional logic to resolve the ambiguity.

For example:

We could use an IN clause in our case statements instead of a simple equality check:
- If there are multiple rows with the same minimum value but different dates, we would need to choose one of these dates. Using an IN clause helps us do this by allowing us to specify all possible values for that date.
Alternatively, if we know which row corresponds to the desired date (for example, based on some additional information), we could modify our case statements to explicitly return the desired date.

However, without further context or information about how to resolve these ambiguities, we will stick with using an IN clause in this solution.

Last modified on 2023-05-27