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Databases Querying Relational Databases Subqueries Review and Practice

drstrangequark
drstrangequark
8,273 Points
Posted by drstrangequark
drstrangequark
8,273 Points

Still having issues with Joins

In SQL Playgrounds for Beginner SQL -> Querying Relational Databases -> Subqueries -> Review and Practice, the second assignment is:

-- Generate a report that lists a patron's first name, email and loan count for loans that haven't been returned.

This time, I'm unsure what I'm doing wrong. It tells me there is a syntax error near "INNER" but I'm still pretty lost. My query is:

SELECT patrons.first_name, patrons.last_name, patrons.email, COUNT(*) FROM ( SELECT * FROM loans_north UNION ALL SELECT * FROM loans_south ) AS total_loans WHERE returned_on IS NULL INNER JOIN patrons ON patrons.id = total_loans.id GROUP BY patron_id;

drstrangequark
drstrangequark
8,273 Points
drstrangequark
drstrangequark
8,273 Points

NEVERMIND! I think I got it:

SELECT p.id AS "Patron ID", p.first_name AS "First Name", p.last_name AS "Last Name", p.email AS "Email", COUNT(*) AS "Unreturned Loan Count" FROM

( SELECT * FROM loans_north WHERE returned_on IS NULL

UNION ALL

SELECT * FROM loans_south WHERE returned_on IS NULL) AS total_loans

INNER JOIN patrons AS p ON p.id = total_loans.patron_id

GROUP BY p.id;

I'm sure that it can be cleaned up a bit but it got me the info I needed!

1 Answer

Steven Parker
Steven Parker
229,644 Points
Steven Parker
Steven Parker
229,644 Points

You were pretty close the first time, and had a good idea about having the WHERE outside of the subquery but it needed to come after the JOIN:

-- Generate a report that lists a patron's first name, email and loan count
-- for loans that haven't been returned.
SELECT first_name, email, COUNT(1) AS "loan count"
FROM (SELECT * FROM loans_north UNION ALL SELECT * FROM loans_south) AS total_loans 
INNER JOIN patrons ON patrons.id = total_loans.patron_id
WHERE returned_on IS NULL
GROUP BY patron_id;