Tutorials Programming Tutorials, Guides & Best Practices

• Select Type: Type of query (simple, primary, subquery, etc.).
• Table: The table accessed by the query.
• Type: Type of access (e.g., index, ALL, ref, const).
• Possible Keys: Indexes considered by the optimizer.
• Key: Index used for the query.
• Rows: Estimated number of rows examined.
• Extra: Additional information, such as whether a temporary table or file sort is used.

Use EXPLAIN to identify full table scans (type = ALL) and optimize them by adding appropriate indexes.

The LEFT JOIN returns all records from the left table and the matched records from the right table. Unmatched records from the right table are returned as NULL.

SELECT columns
FROM table1
LEFT JOIN table2
ON table1.common_column = table2.common_column;