SQL JOIN从左表获取数据，即使所有右列都匹配

Question

I am using PostgreSQL 9.5. I have tow tables

EmployeeReportMarch
EmployeeId      Bonus      Day
---------------------------------
1               10000      23

EmployeeReport20152016
EmployeeId      Bonus      Month
---------------------------------
1               10000      02
1               10000      03

EmployeeReport
EmployeeId      Bonus      FiscalYear
---------------------------------
1               100000      20152016
1               90000       20162017

Calendar
Day_Id          Day    Month     FiscalYear
--------------------------------------------
2015-03-21      21     3         20152016
2015-03-22      22     3         20152016
2015-03-23      23     3         20152016
2015-03-24      24     3         20152016
2015-03-25      25     3         20152016

Calendar table is from '2010-01-01' to '2016-12-31' . Fiscal Year is from 1st April to 31st March.

When I am joining both table to get data from

SELECT e.EmployeeeId, e.Bonus
FROM Employee e INNER JOIN Calendar c ON e.Day = c.Day
WHERE c.Day_Id BETWEEN '2015-02-01'::Date AND '2015-03-31'::Date;

Output :

EmployeeId     Bonus     Day_Id
-------------------------------------
1              10000     2015-02-23
1              10000     2015-03-23

SELECT e.EmployeeeId, e.Bonus
FROM Employee e INNER JOIN Calendar c ON e.Month = c.Month
WHERE c.Day_Id BETWEEN '2015-02-01'::Date AND '2015-03-31'::Date;

Output :

EmployeeId     Bonus
--------------------
1              100000
1              100000

--59 Rows (2 Month rows)

SELECT e.EmployeeeId, e.Bonus
FROM Employee e INNER JOIN Calendar c ON e.FiscalYear = c.FiscalYear
WHERE c.Day_Id BETWEEN '2015-02-01'::Date AND '2015-03-31'::Date;

Output :

EmployeeId     Bonus
--------------------
1              10000
1              10000

--365 Rows

But there is only one record left in table. I have tried with OUTER JOIN also but the result was same any idea How to do it.

Answer 1

join combines matching rows from both tables. Since the day in employee table matches twice with day in calendar table within the range given in between statement, that is why two rows in output. I think if you use any join, it will give two rows in output.

Answer 2

It's problem of architecture, not JOIN statement...

Your table Calendar have Primary Key column Day_Id , but You trying to JOIN it by column Day, which is not unique. There are a lot of records with Day = 23 (12 per Year, to be exact). So, You get 1 row per each month in Your table.

To fix this problem, You should JOIN it by column Day_Id (or by 3 columns at once: Year + Month + Day).