Maybe there's a better way to do this... I have a table of member friend requests. The columns are request_id, author_id, recipient_id, status(accepted or denied). I also have a members table whose id is linked to the author or recipient. I want to get a list of a member's friends by selecting from the members table and then joining the requests table. Since the member can be either the author or the recipient of any, some, or none of the requests, a simple LEFT JOIN member_requests AS r ON member_id = r.author_id wouldn't work. How can I write a query that will do this?
SELECT
m.member_id, m.display_name
r.author_id, r.recipient_id, r.status
FROM members AS m
LEFT JOIN member_requests AS r ON m.member_id = r.recipient_id
WHERE r.status = 1 --Accepted
ORDER BY m.display_name
You can use an OR in your left join, like so:
LEFT JOIN member_requests AS r
ON m.member_id = r.recipient_id
OR m.member_id = r.author_id
However, your where clause also needs to be altered:
SELECT
m.member_id, m.display_name
r.author_id, r.recipient_id, r.status
FROM members AS m
LEFT JOIN member_requests AS r
ON (m.member_id = r.recipient_id
OR m.member_id = r.author_id)
AND r.status = 1 //Accepted
ORDER BY m.display_name
When you left join table A to table B, and then specify a restriction in your where clause on table B, you convert your left join into an inner join. Typically, a left join to table B would yield some null values, since table A might have records that don't join to table B. But if you say 'where table B.value = x', you restrict your join to only rows in which table A joins to table B, and furthermore to rows in which 'B.value = x'. The join is then evaluated as an inner join, rather than a left outer.
Related
I have a database structure with the following setup:
po: id, stockNumber, factoryId, other columns
order: id, stockNumber, factoryId, other columns
stock_number: id, stockNumber, groupId
factory: id, name, groupId
The important part here is the stock_number/factory tables. The groupId column is just an integer and if two or more rows in the table have the same value then their stock numbers/factory are considered the same. Typically this is used for different sizes of the same product.
What I'd like to do is write a query that will join "order" to "po" through the group of stock_number and factory so I can find orders with no matching po. Also the factory has to match the same way.
I have this query if I have a specific stock number/factory in mind but I'd like to update it to query the whole orders table for me:
SELECT id
FROM order
WHERE
styleNumber IN (SELECT a.stockNumber FROM stock_number a INNER JOIN stock_number b ON a.groupId = b.groupId or a.id = b.id WHERE b.stockNumber = '123')
AND factoryId IN (SELECT a.submitter_id FROM submitter a INNER JOIN submitter b ON a.groupId = b.groupId OR a.submitter_id = b.submitter_id WHERE b.SUBMITTER_ID = 'alpha');
EDIT: I came up with this query which I think might be on the right track. It only joins in the stock number so it doesn't do factory yet. Can anyone confirm if I'm going in the correct direction:
SELECT *
FROM order o
LEFT JOIN stock_number s_o ON o.stockNumber = s_o.stockNumber
LEFT JOIN stock_number s_p ON s_o.groupId = s_p.groupId
LEFT JOIN po p ON s_p.stockNumber = p.stockNumber
WHERE p.id IS NULL;
Just join all the tables.
select o.id
FROM order AS o
JOIN stock_number AS sn ON sn.stockNumber = o.stockNumber
JOIN submitter AS su ON ON o.factoryId = su.submitter_id
You could use an anti-join pattern. In this example, it looks complicated because of the two relationship tables. But a query something like this:
SELECT o.id
, o.stockNumber
, o.factoryId
FROM `order` o
LEFT
JOIN `stock_number` s
ON s.stockNumber = o.stockNumber
LEFT
JOIN `factory` f
ON f.id = o.factoryId
AND f.groupId = s.groupId
LEFT
JOIN `po` p
ON p.stockNumber = s.stockNumber
AND p.factoryId = f.id
WHERE p.id IS NULL
The anti-join pattern is easier to visualize with a simpler example. Say you had the order table (as in your example), and an order_line table, with rows related to the order table by the order_id column.
order_line: id, order_id, othercolumns
To get order along with matching order_line rows:
SELECT o.id AS order_id
, l.id AS line_id
FROM `order` o
JOIN `order_line` l
ON l.order_id = o.id
To include rows from order that don't have any matching rows in order_line, we can use an outer join. We add the LEFT keyword:
SELECT o.id AS order_id
, l.id AS line_id
FROM `order` o
LEFT
JOIN `order_line` l
ON l.order_id = o.id
That gets all rows from order, including rows that don't have a matching row in order_line. The trick now is to exclude all the rows that have a matching row. For any rows that didn't have a match, the columns from order_line will be NULL. So we can add a test in the WHERE clause, to exclude rows that had a match.
SELECT o.id AS order_id
, l.id AS line_id
FROM `order` o
LEFT
JOIN `order_line` l
ON l.order_id = o.id
WHERE l.order_id IS NULL
That gets us rows from order that don't have a matching row in order_line.
We can use this same pattern in a more complicated query. We use outer join operations, and rows from order that don't have a matching row in po will have NULL values for the columns from po.
I have three tables, libraryitems, copies and loans.
A libraryitem hasMany copies, and a copy hasMany loans.
I'm trying to get the latest loan entry for a copy only; The query below returns all loans for a given copy.
SELECT
libraryitems.title,
copies.id,
copies.qruuid,
loans.id AS loanid,
loans.status,
loans.byname,
loans.byemail,
loans.createdAt
FROM copies
INNER JOIN libraryitems ON copies.libraryitemid = libraryitems.id AND libraryitems.deletedAt IS NULL
LEFT OUTER JOIN loans ON copies.id = loans.copyid
WHERE copies.libraryitemid = 1
ORDER BY copies.id ASC, loans.createdAt DESC
I know there needs to be a sub select of some description in here, but struggling to get the correct syntax. How do I only return the latest, i.e MAX(loans.createdAt) row for each distinct copy? Just using group by copies.id returns the earliest, rather than latest entry.
Image example below:
in the subquery , getting maximum created time for a loan i.e. latest entry and joining back with loans to get other details.
SELECT
T.title,
T.id,
T.qruuid,
loans.id AS loanid,
loans.status,
loans.byname,
loans.byemail,
loans.createdAt
FROM
(
SELECT C.id, C.qruuid, L.title, MAX(LN.createdAt) as maxCreatedTime
FROM Copies C
INNER JOIN libraryitems L ON C.libraryitemid = L.id
AND L.deletedAt IS NULL
LEFT OUTER JOIN loans LN ON C.id = LN.copyid
GROUP BY C.id, C.qruuid, L.title) T
JOIN loans ON T.id = loans.copyid
AND T.maxCreatedTime = loans.createdAt
A self left join on loans table will give you latest loan of a copy, you may join the query to the other tables to fetch the desired output.
select * from loans A
left outer join loans B
on A.copyid = B.copyid and A.createdAt < B.createdAt
where B.createdAt is null;
This is your query with one simple modification -- table aliases to make it clearer.
SELECT li.title, c.id, c.qruuid,
l.id AS loanid, l.status, l.byname, l.byemail, l.createdAt
FROM copies c INNER JOIN
libraryitems li
ON c.libraryitemid = li.id AND
li.deletedAt IS NULL LEFT JOIN
loans l
ON c.id = l.copyid
WHERE c.libraryitemid = 1
ORDER BY c.id ASC, l.createdAt DESC ;
With this as a beginning let's think about what you need. You want the load with the latest createdAt date for each c.id. You can get this information with a subquery:
select l.copyid, max(createdAt)
from loans
group by l.copyId
Now, you just need to join this information back in:
SELECT li.title, c.id, c.qruuid,
l.id AS loanid, l.status, l.byname, l.byemail, l.createdAt
FROM copies c INNER JOIN
libraryitems li
ON c.libraryitemid = li.id AND
li.deletedAt IS NULL LEFT JOIN
loans l
ON c.id = l.copyid LEFT JOIN
(SELECT l.copyid, max(l.createdAt) as maxca
FROM loans
GROUP BY l.copyid
) lmax
ON l.copyId = lmax.copyId and l.createdAt = lmax.maxca
WHERE c.libraryitemid = 1
ORDER BY c.id ASC, l.createdAt DESC ;
This should give you the most recent record. And, the use of left join should keep all copies, even those that have never been leant.
I want to retrieve rows from 3 tables - LikeTable, ClosetTable, FollowTable to display the activities of all the users a person is following. This is for a website. ClosetTable is the table for the user's products. LikeTable is the table for the likes of the products and Followtable for the following and followers.
Currently, I have
SELECT c.user, c.productName
FROM ClosetTable AS c
LEFT JOIN FollowTable AS f ON c.user = f.isFollowed
WHERE f.follows = 'tony'
This returns the rows of the person, 'tony' is following together with the productname. However, I want rows from the LikeTable and FollowTable in the same manner altogether. So can you suggest a way to do only 1 query to get rows from all 3 tables?
You can try something that look like this :
SELECT c.user, c.productName
FROM FollowTable AS f
INNER JOIN ClosetTable AS c ON c.user = f.isFollowed
INNER JOIN LikeTable AS l ON c.user = l.userliked
WHERE f.follows = 'tony'
I do some assumption for fields name since you didn't provide the structures of your tables.
Also, I suggested that you put the FollowTable in the FROM clause and then join it to ClosetTable since you put f.follows = 'tony' in your WHERE clause.
Otherwise, remove the WHERE clause and put the condition in the INNER JOIN.
Something like :
LEFT JOIN FollowTable AS f ON c.user = f.isFollowed AND f.follows = 'tony'
More JOINs:
SELECT c.user, c.productName
FROM ClosetTable AS c
LEFT JOIN FollowTable AS f ON c.user = f.isFollowed
JOIN LikeTable as l on c.user = l.liked
WHERE f.follows = 'tony'
or something like that. You aren't restricted to one JOIN.
Is it possible to INNER JOIN a MySQL query to achieve this result?
I have a table with Strategies and a table with Members. The Strategy table holds the ID of the author that corresponds to their ID in the Member table and the ID of an author that updated the existing author's work. Is it possible to grab a reference to both of these people at the same time? Something like the following, which returns no errors, but also no results...
SELECT * FROM Strategies
INNER JOIN Members AS a
INNER JOIN Members AS b
WHERE Strategies.ID='2'
AND Strategies.AuthorID = a.ID
AND Strategies.UpdateAuthorID = b.ID
Use a LEFT JOIN:
SELECT
s.*,
a.Name AS MemberName,
b.Name AS UpdatedMemberName
FROM Strategies AS s
LEFT JOIN Members AS a ON s.AuthorID = a.ID AND s.ID = 2
LEFT JOIN Members AS b ON s.UpdateAuthorID = b.ID AND s.ID = 2 ;
If you want them in one column use COALESCE:
SELECT
s.*,
COALESCE(a.Name, b.Name) AS MemberName
FROM Strategies AS s
LEFT JOIN Members AS a ON s.AuthorID = a.ID AND s.ID = 2
LEFT JOIN Members AS b ON s.UpdateAuthorID = b.ID AND s.ID = 2
SELECT toD.dom_url AS ToURL,
fromD.dom_url AS FromUrl,
rvw.*
FROM reviews AS rvw
LEFT JOIN domain AS toD
ON toD.Dom_ID = rvw.rev_dom_for
LEFT JOIN domain AS fromD
ON fromD.Dom_ID = rvw.rev_dom_from
if domain is table name
I am trying to fetch out records from left table only those records which are not in right table. I have near about 5000 records in the left table. Similarly, the same thing is need to be done on more than 3-4 tables. I have to find out records from first table that are not in rest of my 3-4 tables. The same primary or foreign key concept is working their.
My first attempt taking two tables fetch out 5,70,000 records from original 5k records. Repeating the record.
SELECT m.* FROM members m, pinnumber p where p.pinmemberid != m.memberid
My second attempt also made my-sql browser hang.
SELECT m.* FROM members m
LEFT JOIN pinnumber p ON p.pinmemberid != m.memberid
LEFT JOIN customer c ON m.memberid != c.memberid
My third attempt is also making a lot of time
SELECT * FROM members m
WHERE 1=1 AND AND not exists ( select 1 from pinnumber p where 1=1 And
p.pinmemberid = m.memberid AND p.pinproductid LIKE '%Remit%')
AND not exists ( select 1 from customer c where 1=1 and c.card_name is not null AND m.memberid = c.memberid )
Please suggest me what to do with this. If I need to put non-equi join on this.
This query will try to join the first table members with tables pinnumber and customer:
SELECT m.*
FROM
members m
LEFT JOIN pinnumber p ON p.pinmemberid = m.memberid
LEFT JOIN customer c ON m.memberid = c.memberid
WHERE p.pinmemberid is null and c.memberid is null
since we are using a left join, the query will return all rows of members but if the join doesn't succeed pinmemberid and memberid will be null. The rows in which those values are null are the ones that doesn't exist in the right tables. You might want to use OR instead of AND.
If you use a join with != condition, you will get all rows of the first table multiplied for all rows of the second table, except the ones that have the same id.
Another way to get the same result is this:
SELECT members.*
FROM members
WHERE
memberid not in (select pinmembedid from pinnumber)
and memberid not in (select memberid from customer)
(substitute AND with OR if you wish). NOT IN clause might be a little slower, but it's easier to understand.
By the way, what you were trying to do is:
SELECT m.* FROM members m
LEFT JOIN pinnumber p ON p.pinmemberid = m.memberid
LEFT JOIN customer c ON m.memberid = c.memberid
where p.pinmemberid is null and c.pinmemberid is null
The left join produces NULL values when there are no records in the right table. You can filter for these after the join.
I hope this command will produce your desired output..
SELECT m.* FROM members m LEFT JOIN pinnumber p ON p.pinmemberid = m.memberid where m.memberid is null;