Understand the critical rule: NULL never matches NULL in join conditions (NULL = NULL returns NULL, not TRUE)

Recognize that rows with NULL foreign keys are excluded from INNER JOIN results

Learn why LEFT JOIN preserves rows with NULL foreign keys while INNER JOIN discards them

Use IS NULL to find rows with NULL foreign keys (orphaned records)

Distinguish between NULL as "missing data" vs NULL as "no relationship"

Handle NULL values correctly in join conditions to avoid losing data

Identify data quality issues caused by NULL foreign keys in production databases

Apply COALESCE and IFNULL to provide default values for NULL columns in join results

Understand the critical rule: NULL never matches NULL in join conditions (NULL = NULL returns NULL, not TRUE)

Recognize that rows with NULL foreign keys are excluded from INNER JOIN results

Learn why LEFT JOIN preserves rows with NULL foreign keys while INNER JOIN discards them

Use IS NULL to find rows with NULL foreign keys (orphaned records)

Distinguish between NULL as "missing data" vs NULL as "no relationship"

Handle NULL values correctly in join conditions to avoid losing data

Identify data quality issues caused by NULL foreign keys in production databases

Apply COALESCE and IFNULL to provide default values for NULL columns in join results