SELECT students.`name` FROM students, classes WHERE classes.id = students.class_id;
SELECT s.id, s.name, s.class_id, c.name class_name, s.gender, s.score
FROM students s
INNER JOIN classes c
ON s.class_id = c.id;
直接sql先笛卡尔，再筛选，表越大效率越有问题
内连接只在主表加指定列，不存在效率问题
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SELECT students.`name` FROM students, classes WHERE classes.id = students.class_id;
SELECT s.id, s.name, s.class_id, c.name class_name, s.gender, s.score
FROM students s
INNER JOIN classes c
ON s.class_id = c.id;