How to scan through multiple items in MySQL? - mysql

Question

I want to select all elements from the element table, but with an extra column stating whether or not ANY of their comments have admin status. A comment has admin status if the user that posted the status' admin column is 1.
I don't know how I'd scan through each comment for every element being queried.
In another part of the program, there is a query to draw down all comments of a single issue, and I was able to reason how to determine admin status there, but I can't think of a way to do it in a query that pulls down more than one issue.
SELECT comments.id, comments.elementID, comments.googleID, comments.time, comments.body, users.name, users.admin
FROM comments
LEFT JOIN users ON comments.googleID = users.googleID
WHERE comments.elementID = ? AND comments.approved = 1
ORDER BY comments.time DESC

Answer 1

To know which elements have an comment from an admin user, you could simply do:
SELECT C.elementID, MAX(U.admin) AS admin
FROM comments C
LEFT JOIN users U ON C.googleId = U.googleID
WHERE C.approved = 1
GROUP BY C.elementID;
and then to get all elements with the additional admin column:
SELECT E.*, CASE WHEN A.admin = 1 THEN 1 ELSE 0 END AS admin
FROM elements E
LEFT OUTER JOIN
(SELECT C.elementID, MAX(U.admin) AS admin
FROM comments C
LEFT JOIN users U ON C.googleId = U.googleID
WHERE C.approved = 1
GROUP BY C.elementID) A
ON E.id = A.elementID

Answer 2

You can use a case expression with an exists predicate and a correlated subquery like so:
SELECT
e.*,
CASE WHEN EXISTS (
SELECT 1
FROM comments c
JOIN users u ON c.googleID = u.googleID
WHERE e.googleID = c.googleID AND u.admin = 1
) THEN 'Yes' ELSE 'No' END AS AdminComment
FROM elements e;
Or you could express it using joins:
SELECT
e.*,
CASE WHEN u.admin IS NOT NULL THEN 'Yes' ELSE 'No' END AS AdminComment
FROM elements e
LEFT JOIN comments c ON e.googleID = c.googleID
LEFT JOIN users u ON c.googleID = u.googleID AND u.admin = 1