How to write multiple joins in MySQL - mysql

Question

I am trying to a join a table with a second table created from another join. I'm not sure if i'm doing this correctly. I am receving an error code that "Every derived table must have its own alias". Creating an alias is not something I have done before. Could you please let me know how to adjust my query? Thank you.
Select
questionnaire_status.*
FROM
questionnaire_status
LEFT JOIN
(SELECT
rep_list.Last_Name,
rep_list.First_Name,
dept_codes.dept_name
FROM
rep_list
Left Join
dept_codes
ON
rep_list.dept =dept_codes.dept_id
)
ON
questionnaire_status.RR = rep_list.RR
WHERE
questionnaire_Status.Attestation_submitted = '0'

Answer 1

You are currently using subquerys, something you should not do unless absolutely necessary. (<- Performance is very bad)
Better are Multiple Joins are like this:
SELECT
questionnaire_status.*,
rep_list.Last_Name,
rep_list.First_Name,
dept_codes.dept_name
FROM
questionarrie_status
LEFT JOIN
rep_list
ON
questionnaire_status.RR = rep_list.RR
LEFT JOIN
dept_codes
ON
rep_list = dept_codes.dept_id
LEFT JOIN
questionarrie_status
WHERE
questionnaire_Status.Attestation_submitted = '0'

Answer 2

Use the as keyword to alias a subquery thus:
Select
questionnaire_status.*
FROM
questionnaire_status
LEFT JOIN
(SELECT
rep_list.Last_Name,
rep_list.First_Name,
rep_list.RR,
dept_codes.dept_name
FROM
rep_list
Left Join
dept_codes
ON
rep_list.dept =dept_codes.dept_id
) as tbl_alias
ON
questionnaire_status.RR = tbl_alias.RR
WHERE
questionnaire_Status.Attestation_submitted = '0
but you need to add the join column to the inner select as above

Answer 3

To get the results I think you're trying to get, all you have to do is this:
Select
questionnaire_status.*
FROM
questionnaire_status
LEFT JOIN rep_list ON questionnaire_status.RR = rep_list.RR
LEFT JOIN dept_codes ON rep_list.dept = dept_codes.dept_id
As for the aliases, when you do this you're creating the alias "me":
SELECT * FROM myTable me;
MySQL was complaining about your subquery not having an alias:
(SELECT
rep_list.Last_Name,
rep_list.First_Name,
dept_codes.dept_name
FROM
rep_list
Left Join
dept_codes
ON
rep_list.dept =dept_codes.dept_id
)
^^^ It wanted an alias here, to cover the subquery
So the next time you have a subquery, do something like this:
(SELECT
rep_list.Last_Name,
rep_list.First_Name,
dept_codes.dept_name
FROM
rep_list
Left Join
dept_codes
ON
rep_list.dept =dept_codes.dept_id
) REP
^^^ MySQL has an alias (named REP) so it's happy now