SQL LEFT JOIN

LEFT JOIN 关键字从左表（table1）返回所有的行，即使右表（table2）中没有匹配。如果右表中没有匹配，则结果为 NULL。

SELECT column_name(s)FROM table1LEFT JOIN table2ON table1.column_name=table2.column_name;

以上一篇的student和class表为例

SELECT student.stuid,class.classname,student.name FROM student LEFT JOIN class ON student.name=class.classteacher;

SELECT student.stuid,class.classname,student.name FROM class LEFT JOIN student ON student.name=class.classteacher;