DB2筛选数据行并获取字符串排序后的最新条目

First, let's recap your requirements clearly:

• When a combination of Nr and Key appears multiple times:
  • Append ".." to the Nr value
  • Only keep the lexicographically largest (latest when sorted) String value from the group
• When the combination is unique:
  • Keep the original Nr, Key, and String values as-is

Input Table (TableA) #

Expected Output #

Complete SQL Query #

WITH tmp AS (
    SELECT 
        Nr,
        Key,
        COUNT(*) AS occurrence_count,
        MAX(String) AS latest_sorted_string
    FROM TableA
    GROUP BY Nr, Key
)
SELECT 
    CASE 
        WHEN occurrence_count > 1 THEN CONCAT(CAST(Nr AS VARCHAR), '..') 
        ELSE CAST(Nr AS VARCHAR) 
    END AS Nr,
    Key,
    latest_sorted_string AS String
FROM tmp;

How This Works #

1. CTE (tmp):

   • Groups the data by Nr and Key to identify duplicate combinations
   • COUNT(*) tracks how many times each (Nr, Key) pair appears
   • MAX(String) grabs the lexicographically largest string from each group (which matches your requirement of keeping the "latest sorted value")

2. Main Query:

   • Uses a CASE statement to append ".." to Nr only if the pair has multiple occurrences
   • Casts Nr to a string type first to safely concatenate the suffix
   • Selects the original Key and the precomputed latest string from the CTE

This approach efficiently handles both unique and duplicate groups in a single pass over the data, ensuring your expected output is generated correctly.

内容的提问来源于stack exchange，提问作者Viking