sql - 需要帮助识别表中的重复项

Question

我有什么:

1. data_source_1 表
2. data_source_2 表
3. data_sources_view 查看

关于表格:

data_source_1:

没有重复:

db=# select count(*) from (select distinct * from data_source_1);
count 
--------
543243
(1 row)

db=# select count(*) from (select * from data_source_1);
count 
--------
543243
(1 row)

data_source_2:

没有重复:

db=# select count(*) from (select * from data_source_2);
count 
-------
5304
(1 row)

db=# select count(*) from (select distinct * from data_source_2);
count 
-------
5304
(1 row)

data_sources_view:

有重复:

db=# select count(*) from (select distinct * from data_sources_vie);
count 
--------
538714
(1 row)

db=# select count(*) from (select * from data_sources_view);
count 
--------
548547
(1 row)

View 很简单:

CREATE VIEW data_sources_view
AS SELECT * 
FROM (
      (
       SELECT a, b, 'data_source_1' as source
       FROM data_source_1
      )
      UNION ALL 
      ( 
       SELECT a, b, 'data_source_2' as source
       FROM data_source_2
      )
);

我想知道的:

• 在源表没有重复项的 View 中怎么可能有重复项 + 'data_source_x' 作为源 消除了重叠数据的可能性。
• 如何识别复制品？

我尝试过的:

db# create table t1 as select * from data_sources_view;
SELECT
db=# 
db=# create table t2 as select distinct * from data_sources_view;
SELECT
db=# create table t3 as select * from t1 minus select * from t2;
SELECT
db=# select 't1' as table_name, count(*) from t1 UNION ALL
db-# select 't2' as table_name, count(*) from t2 UNION ALL
db-# select 't3' as table_name, count(*) from t3;
table_name | count 
------------+--------
t1 | 548547
t3 | 0
t2 | 538714
(3 rows)

数据库:

Redshift (PostgreSQL)

Answer 1

原因是因为你的数据源有多于两列。如果您进行这些计数:

select count(*) from (select distinct a, b from data_source_1);

和

select count(*) from (select distinct a, b from data_source_2);

你应该会发现它们和你在同一张 table 上得到的count(*)不一样。