SQL 语句与性能之联合查询和联合分类查询

select
    *
from t1
left join t2 on t2.sysno =t1.ASysNo
left join t3 on t3.sysno =t2.ASysNo
left join t4 on t4.sysno =t3.ASysNo
left join t5 on t5.sysno =t4.ASysNo
left join t6 on t6.sysno =t5.ASysNo
left join t7 on t7.sysno =t6.ASysNo
where t1.name like '%AAA%'

在没有建任何索引的情况下，模糊查询，这类sql性能最差，这类SQL为联合查询SQL

select
    *
from t1
left join (
   select
      *
   from t2
   left join t3 on t3.sysno =t2.ASysNo
   left join t4 on t4.sysno =t3.ASysNo
) vw_detail on  vw_detail .sysno =t1.ASysNo
left join (
   select
      *
   from t5
   left join t6 on t6.sysno =t5.ASysNo
   left join t7 on t7.sysno =t6.ASysNo
) vw_mm on vw_mm.sysno = t1.SSysNo
where charindex('AAA',t1.name)>0

将SQL用left-join把一类的数据包起来分类关联,去掉like查询，尽量减少where中使用函数,然后对各个表的外键建立索引;按照常理来说；这类SQL查询个人称为联合分类查询SQL,一般按照该规则查询后查询应该很快了

以上SQL仅供参考