从简单实例来看 left join 如何去重

1. 执行语句，创建3张表

CREATE TABLE table_dev1 (
	id serial NOT NULL,
	devicecode text NULL, 		-- 设备id
	openflag int4 NULL 		 	-- 是否启动 1：启动, 0：未启动
);

CREATE TABLE table_dev2 (
	id serial NOT NULL,
	devicecode text NULL, 		-- 设备id
	devicetype int4 NULL, 		-- 设备类型
	vendor text NULL 			-- 厂家名称
);

CREATE TABLE table_dev3 (
	id serial NOT NULL,
	devicecode text NULL, 		-- 设备id
	openflag int4 NULL, 		-- 是否启动 1：启动, 0：未启动
	devicetype int4 NULL, 		-- 设备类型
	vendor text NULL 			-- 厂家名称
);

2. 插入数据到 table_dev1 & table_dev2

insert into table_dev1 (devicecode, openflag)values ('123', 1);
insert into table_dev1 (devicecode, openflag)values ('abc', 0);

insert into table_dev2 (devicecode, devicetype, vendor)values ('123', 100, '路人');
insert into table_dev2 (devicecode, devicetype, vendor)values ('123', 100, '路人');
insert into table_dev2 (devicecode, devicetype, vendor)values ('abc', 101, '未知');

得到表数据

3. 使用 left join 关联表 table_dev1 & table_dev1，并把数据插入到表 table_dev3

1）未去重

insert into table_dev3 (devicecode, openflag, devicetype, vendor) 
select t1.devicecode, t1.openflag, t2.devicetype, t2.vendor 
from table_dev1 t1 
left join 
table_dev2 t2  
on t1.devicecode = t2.devicecode 

得到结果 >>

2）去重

insert into table_dev3 (devicecode, openflag, devicetype, vendor) 
select t1.devicecode, t1.openflag, t2.devicetype, t2.vendor 
from table_dev1 t1 
left join 
(select t3.* from 
  (select devicecode,devicetype,vendor,row_number() over (partition by devicecode order by id desc) as rownum from table_dev2) t3 where t3.rownum = 1 ) t2  
 on t1.devicecode = t2.devicecode 

得到结果 >>