Why is my T-SQL LEFT JOIN Returning Fewer Rows Than Expected?

Why is Your T-SQL Left Join Not Producing the Expected Result?

In the provided SQL query, despite employing a LEFT JOIN between two tables (#appSteps and #appProgress), only two rows are returned instead of the expected three. To identify the cause of this unexpected behavior, let's analyze the query and its components.

The LEFT JOIN operation aims to return all rows from the left table (#appSteps), even if they do not have corresponding matches in the right table (#appProgress). However, the query includes a condition in the WHERE clause that filters out the NULL rows from #appSteps:

where s.section is not null

This condition inadvertently limits the result set to rows where #appSteps.section is non-NULL, effectively converting the LEFT JOIN to an INNER JOIN. The rows with NULL values in #appSteps.section are excluded, resulting in the absence of the expected third row.

To rectify this issue, the condition should be moved to the ON condition of the LEFT JOIN:

Left Join #appProgress P   On  S.stepId = P.stepId 
                           And P.appId = 101

By specifying the condition in the ON clause, it becomes part of the join criteria, ensuring that only matching rows from #appSteps are fetched from #appProgress. Consequently, the LEFT JOIN will still retrieve all rows from #appSteps, including those with NULL values, regardless of whether they match #appProgress.

By using this approach, the query will return the desired three rows, as originally intended.

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