SQL query that selects effective costing rate based on charge date - sql

Question

SQL novice here. I'm trying to generate a costing query that outputs employee time card information and calculates cost based on an effective employee costing rate.
My question is similar to the one asked here: Retro-active effective date changes with overlapping dates but I'm not dealing with retro-activity or overlapping date ranges.
Table examples (null values in the rate table indicate current rate):
CREATE TABLE Emp_Rate
(
Emp int,
Rate money,
Rate_Start datetime,
Rate_Exp datetime
)
CREATE TABLE Emp_Time
(
Emp int,
Chrg_Date datetime,
Chrg_Code varchar(10),
Chrg_Hrs decimal(8, 2)
)
Insert into Emp_Rate (Emp,Rate,Rate_Start,Rate_Exp) Values ('1','20','5/1/09','4/30/10')
Insert into Emp_Rate (Emp,Rate,Rate_Start,Rate_Exp) Values ('1','21','5/1/10','4/30/11')
Insert into Emp_Rate (Emp,Rate,Rate_Start,Rate_Exp) Values ('1','22','5/1/11',NULL)
Insert into Emp_Time (Emp,Chrg_Date,Chrg_Code,Chrg_Hrs) Values ('1','5/10/09','B','8')
Insert into Emp_Time (Emp,Chrg_Date,Chrg_Code,Chrg_Hrs) Values ('1','5/10/10','B','8')
Insert into Emp_Time (Emp,Chrg_Date,Chrg_Code,Chrg_Hrs) Values ('1','5/10/11','B','8')
The query (returns dupes caused by multiple rate entries(obviously)):
Select Emp_Time.Emp,
Cast(Emp_Time.Chrg_Date as DATE) as 'Chrg_Date',
Emp_Time.Chrg_Code,
Emp_Time.Chrg_Hrs,
Emp_Rate.Rate,
Emp_Time.Chrg_Hrs * Emp_Rate.Rate as 'Cost'
From Emp_Time inner join
Emp_Rate on Emp_Rate.Emp = Emp_Time.Emp
Order By [Emp],[Chrg_Date]
Desired output:
Emp Chrg_Date Chrg_Code Chrg_Hrs Rate Cost
1 2009-05-10 B 8.00 20.00 160.00
1 2010-05-10 B 8.00 21.00 168.00
1 2011-05-10 B 8.00 22.00 176.00
I've gone around in circles using the Between operator in a sub query to isolate the correct rate based on the charge date, but have not had any luck.
I appreciate any help!

Answer 1

You didn't specify the DBMS type the answer below is for sql-server. I am sure there are other ways to do this but this way will replace the null Rate_Exp date with the current date.
Select et.Emp,
Cast(et.Chrg_Date as DATEtime) as 'Chrg_Date',
et.Chrg_Code,
et.Chrg_Hrs,
er.Rate,
et.Chrg_Hrs * er.Rate as 'Cost'
From Emp_Time et
inner join
(
SELECT Emp
, Rate
, Rate_Start
, CASE
WHEN Rate_Exp is Null
THEN Convert(varchar(10), getdate(), 101)
ELSE Rate_Exp
END as Rate_Exp
FROM Emp_Rate
)er
on er.Emp = et.Emp
WHERE (et.Chrg_Date BETWEEN er.Rate_Start AND er.Rate_Exp)
Order By et.Emp,et.Chrg_Date
OR use the CASE Statement in your WHERE Clause:
Select et.Emp,
Cast(et.Chrg_Date as DATEtime) as 'Chrg_Date',
et.Chrg_Code,
et.Chrg_Hrs,
er.Rate,
et.Chrg_Hrs * er.Rate as 'Cost'
From Emp_Time et
inner join Emp_Rate er
on er.Emp = et.Emp
WHERE (et.Chrg_Date
BETWEEN er.Rate_Start
AND CASE WHEN er.Rate_Exp Is Null
THEN Convert(varchar(10), getdate(), 101)
ELSE er.Rate_Exp END)