- 两个或多个表关联，没写where条件，大量的笛卡尔值，严重时会导致数据库有问题。
select * from a, b left join c on b.id = c.id left join d on c.id = d.id
- 多表关联查询，where条件使用各种函数，导致索引无效。数据量如果打起来以后会导致查询像蜗牛一样。
select trim(d.circuitid) as circuitid, trim(d.Code) as Code, trim(d.Oldname) as Oldname, trim(d.Circuittype) as Circuittype, trim(d.Status) as Status, trim(d.customerid) as acustomerid, trim(d.alinkman) as alinkman, trim(d.zlinkman) as zlinkman, trim(d.alinkmantel) as alinkmantel, trim(d.zlinkmantel) as zlinkmantel, trim(d.busi_no) as busi_no, trim(d.service_serial) as service_serial, trim(a.region_ID) as regionid , 2 as SORTID from test1 a,test2 b,test3 c,test4 d where trim(a.room_id)=trim(b.roomID) and trim(b.neid)=trim(c.neid) and (trim(c.portid)=trim(d.aendname) or trim(c.portid)=trim(d.zendname))
本文转自秋楓博客园博客，原文链接：http://www.cnblogs.com/rwxwsblog/p/4507389.html，如需转载请自行联系原作者
