Join two table with column values based on condition being met

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Summary

The problem at hand is trying to join two tables, tbRecord and tbInspectedPart, in a way that avoids duplicate records. The current query is producing duplicates when joining these tables due to their 1: relationship. The goal is to create a flattened view that combines data from both tables without duplication.

Root Cause

The root cause of the issue is the 1: relationship between tbRecord and tbInspectedPart. When joining these tables, each record in tbRecord can match multiple records in tbInspectedPart, resulting in duplicate records in the output. The current query uses SELECT DISTINCT to try to eliminate duplicates, but this is not sufficient because the joined data includes columns that can have different values for each matching record in tbInspectedPart.

Why This Happens in Real Systems

This issue occurs in real systems when:

  • There are multiple relationships between entities, and the query tries to join them in a way that doesn’t account for these relationships.
  • The data model is not properly normalized, leading to redundant data and difficulties in querying.
  • The query logic is flawed, failing to correctly handle the relationships between tables.

Real-World Impact

The impact of this issue includes:

  • Duplicate data in the output, which can lead to incorrect analysis or reporting.
  • Performance issues, as the query may need to process a large amount of duplicate data.
  • Difficulty in maintaining the query, as changes to the data model or query logic can introduce new duplicates or other issues.

Example or Code

SELECT 
    r.id, 
    r.Status, 
    r.Quantity, 
    ins.SplitQty, 
    ins.DispoId
FROM 
    dbo.tbRecords r
INNER JOIN 
    dbo.tbInspectedParts ins ON r.id = ins.tbRecord_id

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Re-evaluating the data model to ensure it is properly normalized and supports the required queries.
  • Using aggregate functions to combine data from tbInspectedPart into a single row per tbRecord.
  • Applying filters to eliminate unnecessary data and reduce the number of duplicates.
  • Optimizing the query to improve performance and reduce the impact of duplicates.

Why Juniors Miss It

Juniors may miss this issue because:

  • They lack experience with complex data models and queries.
  • They don’t fully understand the relationships between tables and how they impact the query.
  • They focus on the immediate problem rather than taking a step back to evaluate the overall data model and query logic.
  • They don’t test thoroughly, failing to catch duplicate data or other issues that can arise from flawed query logic.