Querying of off a query MYSQL - mysql

Question

I'be been googling around about nested queries but can't find anything that I can grasp about how to go about this particular operation.
First, I'll show you my DB schema
CREATE TABLE slsemp
( empID char(4) NOT NULL,
empname varchar(50) NOT NULL,
prtime enum('yes','no') NOT NULL, # we can only accept yes or no values to the part-time employee indicator
RegionID char(2) NOT NULL, # enums are often used for boolean values in a BD
PosID char(4) NOT NULL,
PRIMARY KEY (empID),
FOREIGN KEY (regionID) REFERENCES region (RegionID),
FOREIGN KEY (PosID) REFERENCES slspos(PosID));
# create the sales transactions table
CREATE TABLE slstranx
( tranxID int(10) NOT NULL AUTO_INCREMENT, #starts at a certain number, then increments accordingly
empID char(4) NOT NULL,
ProdID char(3) NOT NULL,
Qty int(5) NOT NULL,
Unitprice Decimal(5,2) NOT NULL, # please note we are capturing the unit price at the transactional level in this case
SAmt Float(10,2), #store up to 10 (TOTAL?) with 2 digits past decimal point
SalesDate date, # 'date' data type is organized as follows YYYYMMDD. You need to make sure that raw data contains the DATE in the YYYYMMDD format
# For example 20150915
PRIMARY KEY (tranxID),
FOREIGN KEY (ProdID) REFERENCES product (ProdID),
FOREIGN KEY (empID) REFERENCES slsemp (empID));
Now, I want to find employees that are in the west region that haven't made any sales. I figured I'd do this via a left outer join between the two tables then query the resulting table based off of a null tranx ID. I've got it most of the way there, here's my query:
SELECT e.empID, t.tranxID, e.RegionID
FROM slsemp e LEFT OUTER JOIN slstranx t ON e.empID=t.empID
WHERE e.RegionID='WS'
My question is, how do I query based of the criteria of this resultant table. If I could do that, I simply would need a selection with criteria of slstranxID=null.

Answer 1

You can use left join adding where slstranx.empID is null
select distinct empID, empName
from slsemp
left join slstranx on slsemp.empID = slstranx.empID
where slsemp.RegionID = 'WS'
and slstranx.empID is null
if the column from the table in left join is null mean that don't match .. so don't have sales