MySQL JOIN Operations: From Inner Join to Outer Join, A Beginner's Easy Guide

MySQL's JOIN operations are used to combine data from two tables (e.g., a student table and a score table). The core types and their characteristics are as follows: **INNER JOIN**: Returns only matching records from both tables (e.g., Xiaoming, Xiaohong, Xiaogang). The `ON` clause must specify the join condition (e.g., `students.id = scores.student_id`); otherwise, a Cartesian product (incorrect result) will be generated. **LEFT JOIN**: Preserves all records from the left table (student table). If there is no match in the right table (score table), `NULL` is filled (e.g., Xiaoqiang has no score). It is suitable when you need to retain all data from the main table. **RIGHT JOIN**: Preserves all records from the right table (score table). If there is no match in the left table, `NULL` is filled (e.g., scores for student_id=5). It is suitable when you need to retain all data from the secondary table. **FULL JOIN**: Not supported in MySQL. It can be simulated using `LEFT JOIN + UNION`, which includes all students and scores, with `NULL` filling in non-matching parts. Note: The `ON` condition must be written; to filter students with no scores, use `WHERE scores.score IS NULL`; avoid incorrect join conditions that lead to data errors. Core logic: "Retain all from the left table,"