n1n2 FROM table1 a LEFT JOIN table2 b ON a.id = b.id;
n1n2是要查询的列名，id是这两个表中共有的列名。
n1n2n3 FROM table1 a LEFT JOIN table2 b ON a.id = b.id LEFT JOIN table3 c ON a.id = c.id;
n1n2n3是要查询的列名，id是这三个表中共有的列名。
6. 总结
