使用left join比直接使用where速度快的原因

多表使用left join只是把主表里的所有数据查询出来，其他表只查询表中的符合条件的某一条记录，所以速度非常快；而多表使用where内联，是把所有表的数据全查出来，然后进行比对，所以速度非常慢。

使用left join要注意确定哪一张表是主表，如果无法确定主表，则选择哪张表查询的字段最多，就把哪张表作为主表。

示例如下：

使用left join,同样的数据量，时间不到1秒钟！

SELECT a.projectno,MAX(a.projectname) projectname,max(a.projectMoney) projectMoney,
max(a.projectLimitYear) projectLimitYear,max(a.monthcharge) monthcharge,
max(c.orgname) orgname,max(d.businesstypename) businesstypename,max(e.name) name,
max(b.dicvalue) dicvalue,
min(CONVERT(varchar(100),DATEADD("DAY",jbl.DelayDays,jbl.ReportTime),23)) as period,
max(f.fiveleveltype) fiveleveltype,max(a.ProjectInfoId) ProjectInfoId,
max(g.FlowRunId) FlowRunId
FROM
(select ProjectInfoId,ProjectNo,Status,OrgId,TypeID,UserAId,IsDelete,ProjectName,ProjectMoney,ProjectLimitYear,MonthCharge from jt_biz_projectinfo) a
LEFT JOIN
(select b.* from (select MAX(id) id, ProjectNo from JT_Biz_Lecture group by ProjectNo) a,
JT_Biz_Lecture b where a.id=b.id) f
on a.projectno=f.projectno
LEFT JOIN
(select DicId,IsDelete,DicValue,DicCode from jt_Base_Dictionary) b
ON a.status=b.dicId
LEFT JOIN
(select ProjectNo,DelayDays,ReportTime from Jt_Biz_LectureTemp) jbl
ON a.projectno=jbl.projectno
LEFT JOIN
(select IsDelete,OrgName,OrgId from jt_base_org) c
ON a.orgid=c.orgid
LEFT JOIN
(select ID,BusinessTypeName from d_businesstype) d
ON a.typeid=d.id
LEFT JOIN
(select UserInfoId,IsDelete,Name,UserName from jt_base_userInfo) e
ON a.useraid=e.userinfoid
LEFT JOIN
(select UserId,PostId,OrgIds from JT_Base_Post_UserOrg) j
ON j.UserId=e.userinfoid
LEFT JOIN
(select PostId from JT_Base_Post) k
ON j.PostId=k.PostId
LEFT JOIN
(select FlowRunId,ProjectId from jt_flow_run) g
ON a.ProjectInfoId=g.ProjectId
LEFT JOIN
(select FlowRunId from JT_Flow_Run_Prcs) h
ON g.FlowRunId = h.FlowRunId
LEFT JOIN
(select OrgId,OrgName from JT_Base_Org) l
ON a.OrgId=l.OrgId
WHERE a.status in(82,83,84) and a.isdelete=0
and b.isdelete=0 and c.isdelete=0 and e.isdelete=0
and (l.OrgId in(null) or e.UserName='chenqf') group by a.ProjectNo;
--加快查询速度之对比

不使用left join,同样数据量，时间大概在50秒的样子！
select a.projectno,max(a.projectname) projectname,max(a.projectMoney) projectMoney,
max(a.projectLimitYear) projectLimitYear,max(a.monthcharge) monthcharge,
max(c.orgname) orgname,max(d.businesstypename) businesstypename,max(e.name) name,
max(b.dicvalue) dicvalue,
min(CONVERT(varchar(100),DATEADD("DAY",jbl.DelayDays,jbl.ReportTime),23)) as period,
max(f.fiveleveltype) fiveleveltype,max(a.ProjectInfoId) ProjectInfoId,
max(g.FlowRunId) FlowRunId
from (select DicId,IsDelete,DicValue,DicCode from jt_Base_Dictionary) b,
(select IsDelete,OrgName,OrgId from jt_base_org) c,
(select UserInfoId,IsDelete,Name,UserName from jt_base_userInfo) e,
(select ID,BusinessTypeName from d_businesstype) d,
(select FlowRunId,ProjectId from jt_flow_run) g,
(select FlowRunId from JT_Flow_Run_Prcs) h,
(select UserId,PostId,OrgIds from JT_Base_Post_UserOrg) j,
(select PostId from JT_Base_Post) k,
(select OrgId,OrgName from JT_Base_Org) l,
(select ProjectNo,DelayDays,ReportTime from Jt_Biz_LectureTemp) jbl,
(select ProjectInfoId,ProjectNo,Status,OrgId,TypeID,UserAId,IsDelete,ProjectName,ProjectMoney,ProjectLimitYear,MonthCharge from jt_biz_projectinfo) a
left join
(select b.* from (select MAX(id) id, ProjectNo from JT_Biz_Lecture group by ProjectNo) a,
JT_Biz_Lecture b where a.id=b.id) f
on a.projectno=f.projectno where a.status=b.dicId and a.status in(82,83,84)
and a.projectno=jbl.projectno and a.orgid=c.orgid and a.typeid=d.id
and a.useraid=e.userinfoid and j.UserId=e.userinfoid and a.isdelete=0
and b.isdelete=0 and c.isdelete=0 and e.isdelete=0 and g.FlowRunId = h.FlowRunId 
and (l.OrgId in(null) or e.UserName='chenqf')
group by a.ProjectNo;