Filter Table Before Applying Left Join
You could also do:
SELECT c.Customer, c.State, e.Entry
FROM Customer AS c
LEFT JOIN (SELECT * FROM Entry WHERE Category='D') AS e
ON c.Customer=e.Customer
SQL Fiddle here
You need to move the WHERE
filter to the JOIN
condition:
SELECT c.Customer, c.State, e.Entry
FROM Customer c
LEFT JOIN Entry e
ON c.Customer=e.Customer
AND e.Category='D'
See SQL Fiddle with Demo