[FAQ Bot] NEW: Why do my SQL LEFT JOIN queries return NULL values in some columns?

_questions/machine-learning-zoomcamp/general/114_8986dd4006_why-left-join-returns-null.md

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+---
+id: 8986dd4006
+question: Why do my SQL LEFT JOIN queries return NULL values in some columns?
+sort_order: 114
+---
+
+- A LEFT JOIN returns all rows from the left table. If there is no matching row in the right table, the columns from the right table are NULL for that row. This is expected SQL behavior and does not indicate a data ingestion or schema problem. It simply means some records do not have corresponding entries in the joined table.
+
+Example:
+
+```sql
+SELECT a.id, a.name, b.value
+FROM left_table a
+LEFT JOIN right_table b ON a.id = b.id;
+```
+
+In the result, rows with no match will have NULL in b.value. If you'd like to exclude rows without matches, use an INNER JOIN or filter:
+
+```sql
+-- Only rows with a match
+SELECT a.id, a.name, b.value
+FROM left_table a
+INNER JOIN right_table b ON a.id = b.id;
+
+-- Or keep all rows but filter out NULLs
+SELECT a.id, a.name, b.value
+FROM left_table a
+LEFT JOIN right_table b ON a.id = b.id
+WHERE b.value IS NOT NULL;
+```
+
+If you want to display a default value instead of NULL, use COALESCE:
+
+```sql
+SELECT a.id, a.name, COALESCE(b.value, 'N/A') AS value
+FROM left_table a
+LEFT JOIN right_table b ON a.id = b.id;
+```