Why is My LEFT JOIN in SQL Returning Unexpected Results?

Understanding LEFT JOIN: Why Your SQL Query Isn't Returning Expected Results

When utilizing a LEFT JOIN in SQL, it's crucial to recognize its unique characteristics and potential pitfalls. This question illustrates an example where a LEFT JOIN query is returning unexpected results, prompting us to delve into the underlying mechanics and resolve the issue.

The LEFT JOIN operation retrieves all rows from the left table (in this case, #appSteps) regardless of whether it matches with the right table (#appProgress). Any non-matching rows in the right table are assigned NULL values. However, when you include the right table in the WHERE clause (p.appId = 101), you essentially transform the query into an INNER JOIN.

The Solution:

To rectify the problem, the predicate that filters the right table (p.appId = 101) needs to be moved from the WHERE clause to the ON condition of the LEFT JOIN statement. This forces the join to filter the right table before performing the join, ensuring that only matching rows are included.

The adjusted SQL statement is as follows:

Select  P.appId, S.stepId, S.section, P.start
From    #appSteps    S   With (NoLock)
Left Join #appProgress P   On  S.stepId = P.stepId 
                           And P.appId = 101
Where   S.section Is Not Null

By adjusting the query to correctly utilize the LEFT JOIN functionality, it will yield the expected results, including all non-null rows from #appSteps and the matching rows from #appProgress, without inadvertently excluding rows due to additional predicates in the WHERE clause.

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