What happens when JOIN keys do not match?

Parthiban Kannan

Updated on January 8, 2026

< 1 min read

When join keys do not match, the result depends on the join type used.
In INNER JOIN, unmatched records are completely excluded.
In LEFT or RIGHT JOIN, unmatched rows appear with NULL values.
In FULL OUTER JOIN, unmatched rows from both sides are still returned.
In my project, mismatched keys highlighted missing master data.
We used this to identify data quality and integration issues.
Handling these cases is important to avoid incorrect reporting.