CPMagical CP Library

Problem solution · SQL

Change Null Values in a Table to the Previous Value

Change Null Values in a Table to the Previous Value: a SQL solution using relational aggregation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Relational aggregation
Source
walkccc LeetCode Solutions
Length
21 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Relational aggregation

For Change Null Values in a Table to the Previous Value, the query transforms and combines relational rows, then filters or aggregates them into the requested result.

  1. Identify the source rows and join keys.
  2. Apply filters before aggregation when possible.
  3. Group, rank, or project the final columns required by the result.

Code notes

  • 21 lines of SQL from the credited upstream file 2388.sql.
  • The implementation keeps its working state in language-native values and containers.
  • No explicit loop blocks detected.

Complexity

Review join cardinality, grouping keys, and available indexes when estimating query cost.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeChange Null Values in a Table to the Previous Value · SQLSQL
Use this to learn the idea, then write your own version.
WITH  CoffeeShoptWithRowNumber AS (    SELECT *, ROW_NUMBER() OVER() AS `row_number`    FROM CoffeeShop  ),  CoffeeShoptWithRowNumberAndGroupId AS (    SELECT      *,      SUM(drink IS NOT NULL) OVER(        ORDER BY `row_number`      ) AS group_id    FROM CoffeeShoptWithRowNumber  )SELECT  id,  FIRST_VALUE(drink) OVER(    PARTITION by group_id    ORDER BY `row_number`  ) AS drinkFROM CoffeeShoptWithRowNumberAndGroupId;

Keep practising

Did this explanation save you time? I'm a Grade 11 student building this free library to make difficult algorithms easier to understand.

Buy me a coffee ↗