select pid,(select pclassname from tproductclass where pclassid =tproduct.pclassid) from tproduct----1select a.pid ,b.pclassname
from tproduct as a,tproductclass as b where a.pclassid=b.pclassid----2select a.pid ,b.pclassname
from tproduct as a left join tproductclass as b
on a.pclassid=b.pclassid----3
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我一直以为1的性能最高,2次之////没想到通过SQLSERVER执行计划对比了一下..
发现2,3性能一样..1的性能最差...我晕/////不知道你们怎么认为?
from tproduct as a,tproductclass as b where a.pclassid=b.pclassid----2select a.pid ,b.pclassname
from tproduct as a left join tproductclass as b
on a.pclassid=b.pclassid----3
-----------------------------
我一直以为1的性能最高,2次之////没想到通过SQLSERVER执行计划对比了一下..
发现2,3性能一样..1的性能最差...我晕/////不知道你们怎么认为?
2是内连接,3是左连接,效率差不了多少
并且第中查询方式还有个限制,就是子查询的返回结果必须小于等于1条;