Oracle关于left join on…and 及 left join on...whe...-白红宇

Oracle关于left join on…and 及 left join on...whe...

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发布时间：2019-06-20

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关于left join...on...and...以及left join...on...where的区别，网上很多的说法是对于left join...on...and...，and所限定的条件只对关联字段起作用，比如select a.* from tmp_table_a a left join tmp_table_b b on a.col1=b.col1 and b.col2=xx,很多人认为条件b.col2=xx是不起作用的。

对于这种说法，我个人是不认同的，至少来讲，这是一种不负责任的说法。以下是我的一些测试例子，从这里大家就可以理解这两个用法的区别。

select count(*) from tb_bo_valusr_new where month=201010 and brand=3;--141858

select count(*) from tb_bo_valusr_new where month=201010;---2281189

select count(distinct usr_nbr) from tb_bo_valusr_new where brand=3;--152110

select count(distinct usr_nbr) from tb_bo_valusr_new where month=201010;--2281189

select count(*) from tb_bo_valusr_new;--4602747

select count(*) from tmp_msy_bj_001;--986843

select count(*) from tmp_msy_bj_001 where if_wlg='是';--272623

--例1

create table tmp_msy_bj_007

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr;

select count(*) from tmp_msy_bj_007;--1957872

select count(distinct usr_nbr) from tmp_msy_bj_007;---986843

select count(*) from (select distinct * from tmp_msy_bj_007);--1024792

--例2

create table tmp_msy_bj_008

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr and b.month=201010;

select count(*) from tmp_msy_bj_008;--986843

select count(distinct usr_nbr) from tmp_msy_bj_008;--986843

select count(*) from (select distinct * from tmp_msy_bj_008);--986843

--例3

create table tmp_msy_bj_006

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr and b.month=201010

where a.if_wlg='是';

select count(*) from tmp_msy_bj_006;--272623

select count(distinct usr_nbr) from tmp_msy_bj_006;--272623

select count(*) from (select distinct * from tmp_msy_bj_006);--272623

--例4

create table tmp_msy_bj_005

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr and b.month=201010 and b.brand=3;

select count(*) from tmp_msy_bj_005;--986843

select count(distinct usr_nbr) from tmp_msy_bj_005;--986843

select count(*) from(select distinct * from tmp_msy_bj_005);--986843

--例5

create table tmp_msy_bj_003

as select

a.usr_nbr,b.cmcc_branch_cd,b.brand from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr and b.brand=3;

select count(*) from tmp_msy_bj_003;--1062507

select count(distinct usr_nbr) from tmp_msy_bj_003;--986843

select count(*) from (select distinct * from tmp_msy_bj_003);--991560

select count(*) from tmp_msy_bj_003 where brand=3;--154124

select count(distinct usr_nbr) from tmp_msy_bj_003 where brand=3;--78460

select count(*) from tmp_msy_bj_003 where brand is null;--908383

--例6

create table tmp_msy_bj_002

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr where b.brand=3;

select count(*) from tmp_msy_bj_002;---154124

select count(distinct usr_nbr) from tmp_msy_bj_002;--78460

select count(*) from(select distinct * from tmp_msy_bj_002);--83177

--例7

create table tmp_msy_bj_011

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a,tb_bo_valusr_new b where a.usr_nbr=b.usr_nbr and b.brand=3;

select count(*) from tmp_msy_bj_011;---154124

select count(distinct usr_nbr) from tmp_msy_bj_011;--78460

select count(*) from(select distinct * from tmp_msy_bj_011);--83177

--例8

create table tmp_msy_bj_009

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr where b.brand=3 and b.month=201010;

select count(*) from tmp_msy_bj_009;---78421

select count(distinct usr_nbr) from tmp_msy_bj_009;--78421

select count(*) from(select distinct * from tmp_msy_bj_009);--78421

--以上各个表的创建时间都在20秒内完成

--例9

create table tmp_msy_bj_004

as select

a.usr_nbr,b.cmcc_branch_cd from

tmp_msy_bj_001 a

left join tb_bo_valusr_new b on a.usr_nbr=b.usr_nbr and a.if_wlg='是';

--运行25分钟都还没有结果

在以上本例子中，b表tb_bo_valusr_new表中每个月都包含当月全量号码,且单月号码是没重复的，而a 表tmp_msy_bj_001没有重复记录。在这样的例子中，如果在left join 之后用 where 指定 b的非连接字段的限制条件，已相当于将A与B等值连接了，很明显，这时的left join已失去意义，left join的本义是保留前面表的所有记录，因此说将后一表中非连接字段的限制条件放在where之后是一种愚蠢的做法。而从上面的例5还可以看出,在left join 后指定and b.column=....起的效果是：将a中符合b条件的记录连接，并保留a中其它不符合的记录一次。因此可以这么理解left join ...on...and...：可以保证保留前面表的全部记录，并可取到后面表中符合指定条件的记录。而出现后面表的and...条件不起作用的情况是：后一表即b表的记录集是a表的一个子集。

再从例9中看出，绝对不要把a的限制条件放在left join on...and...指定，那是找死。

转载于:https://my.oschina.net/xiufeng/blog/134478

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