多表多内连接查询语句
select * from (select * from table1 left join table2 on table1.field1=table2.field1) as table1 left join table3 on table1.field1=table3.field1
select * from (select * from table1 left join table2 on table1.field1=table2.field1) as table1 left join table3 on table1.field1=table3.field1