R) Using join in R - sql

Question

Given database is down below,
> dbReadTable(jamesdb, "EMPLOYEE")
EMP_NO NI_NO NAME AGE DEPT_NO
1 E1 123 SMITH 21 D1
2 E2 159 SMITH 31 D1
3 E3 5432 BROWN 65 D2
4 E5 7654 GREEN 52 D3
> dbReadTable(jamesdb, "DEPARTMENT")
DEPT_NO NAME MANAGER
1 D1 Accounts E1
2 D2 Stores E3
3 D3 Sales E5
> dbReadTable(jamesdb, "PRODUCT")
PROD_NO NAME COLOR
1 p1 PANTS BLUE
2 p2 PANTS KHAKI
3 p3 SOCKS GREEN
4 p4 SOCKS WHITE
5 p5 SHIRTS WHITE
> dbReadTable(jamesdb, "STOCK_TOTAL")
PROD_NO QUANTITY
1 p1 2000
2 p2 1000
3 p3 1500
4 p4 200
5 p5 800
And down below is what I got so far but I think I have a misunderstanding of using join.
How should I fix them?
Retrieve the employment number of the sales department manager.
dbGetQuery(jamesdb, 'SELECT EMPLOYEE.EMP_NO FROM DEPARTMENT JOIN EMPLOYEE
WHERE DEPARTMENT.NAME = "Sales"')
Who works in Department D2?
dbGetQuery(jamesdb, 'SELECT MANAGER FROM DEPARTMENT WHERE DEPT_NO = "D2"')
How many white-colored products are in stock?
dbGetQuery(jamesdb, 'SELECT SUM(QUANTITY) FROM PRODUCT JOIN STOCK_TOTAL
WHERE PRODUCT.COLOR = "WHITE"')

Answer 1

Joins are typically done matching one (or more) fields from one table with a corresponding field(s) of another table. For instance, I'm inferring that DEPARTMENT.MANAGER is actually a foreign key to EMPLOYEE.EMP_NO, so when you join, you should be very specific about that relationship:
SELECT e.EMP_NO
FROM DEPARTMENT d
LEFT JOIN EMPLOYEE e on d.MANAGER = e.EMP_NUM
WHERE d.NAME = "Sales"
Notes:
Many databases allow you to be sloppy, where they will infer field associations (foreign keys) based on common field names. First, I don't like allowing that inference; second, it doesn't work here.
I personally prefer to be explicit about the type of join, whether left join, inner join, etc. It's a style, you may choose just join if you prefer.
I'm introducing table aliases here (d and e), a way to shorten long table names. However, they are stylistic, not required.
I personally dislike databases that lack a dictionary of foreign keys and have unintuitive names to associate them. For instance, I'm inferring from the contents of the tables that DEPARTMENT.MANAGER is linked to EMPLOYEE.EMP_NUM. If I'm wrong on this inference, then answers below are likely skewed.
For your first question, though, I don't know why you need a join: since MANAGER is already the employee number, this should be simply
select d.MANAGER
from DEPARTMENT d
where d.NAME='Sales'
Similarly, your second question needs no join.
select e.*
from EMPLOYEE e
where e.DEPT_NO='D2'
The last one needs a join, and can be done in a number of ways. One such is:
select sum(case when st.Quantity > 0 then 1 else 0 end) as Count
from STOCK_TOTAL st
left join PRODUCT pr on st.PROD_NO=pr.PROD_NO
where pr.COLOR='WHITE'