MySQL Left Join NULL issues

Question

I have the following MySQL code:

Working Query

SELECT name, AVG(q1) AS avg 
FROM respondents 
LEFT JOIN results_new ON respondents.login_id = results_new.company 
WHERE respondents.brand = 'ABC' 
AND results_new.sdate = 'MAY2014' 
GROUP BY name 
ORDER BY avg 
DESC

The above works correctly, it brings back 35 rows and displays the name and average correctly.

Non working query

SELECT name, AVG(q1) AS avg 
FROM respondents 
LEFT JOIN results_new ON respondents.login_id = results_new.company 
WHERE respondents.brand = 'ABC' 
AND results_new.sdate = 'NOV2014' 
GROUP BY name 
ORDER BY avg 
DESC

The above returns zero results - which is correct as there is no data in the table for `NOV2014'.

However, what I expected to happen was the query to return 35 rows with the name and NULL next to each name. Clearly I think my JOIN is incorrect but I cannot work out where I am going wrong.

Any and all advice welcomed.

Answer 1

Your where clause turns your left join into an inner join since you filter on data in the joined table. Put this condition in the on clause of your join.

SELECT name, AVG(q1) AS avg 
FROM respondents 
LEFT JOIN results_new ON respondents.login_id = results_new.company  
                      AND results_new.sdate = 'NOV2014'
WHERE respondents.brand = 'ABC'
GROUP BY hotel_name 
ORDER BY avg DESC