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MySQL Join Query confuse


MySQL Join Query confuse

By : Shyam
Date : November 19 2020, 01:01 AM
To fix this issue I have 3 table as below: , I'd go with sub-selects in this case:
code :
select A.product_desc,
       (select sum(B.qty) from product_sales_branch_a B where B.product_id = A.product_id),
       (select sum(C.qty) from product_sales_branch_b C where C.product_id = A.product_id) 
from product A 
where A.product_id = 'ABC'


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MySQL Slow LEFT JOIN Query when using a derived query for the join

MySQL Slow LEFT JOIN Query when using a derived query for the join


By : Jose Colom
Date : March 29 2020, 07:55 AM
hope this fix your issue Why the nested select? That might be confusing the optimizer. Try eliminating it:
code :
SELECT
  dev_Profile.ID AS pid,
  Name AS username,
  st1.online
FROM
  dev_Profile
  LEFT JOIN dev_User st1 ON st1.ID = dev_Profile.UserID;
How to do this query in MySQL which one should I use using left join or right join or inner join?

How to do this query in MySQL which one should I use using left join or right join or inner join?


By : gigi
Date : March 29 2020, 07:55 AM
it should still fix some issue , You are looking for a left join on the table you allow nulls in:
code :
SELECT tb.BookCode, tb.Title, ta.AuthorID, tb.AuthorShortN 
FROM book AS tb
INNER JOIN bookauthor AS tba ON tba.BookCode=tb.BookCode
LEFT JOIN author AS ta ON tba.AuthorID=ta.AuthorID
Using OR condition + function in join appears to confuse SQL Server's query optimizer

Using OR condition + function in join appears to confuse SQL Server's query optimizer


By : JustSomeDude
Date : March 29 2020, 07:55 AM
this will help First of all, both first_name_alphaonly and nick_name_alphaonly are actually non-persisted computed columns, so all cardinality est are off, and then multiplied.
Then, there are 857.936 individual clustered index seeks on distinct_first_name table, and only after that the filter including the jw function is applied.
How to optimize a MySQL SELECT query with a LEFT JOIN in main query and an INNER JOIN in subquery?

How to optimize a MySQL SELECT query with a LEFT JOIN in main query and an INNER JOIN in subquery?


By : Paul Onema
Date : March 29 2020, 07:55 AM
I wish this helpful for you One possible way to do this would be to use a pair of LEFT joins, and then determine whether to return B.adid depending on whether a match has been found on C:-
code :
SELECT A.*, 
    IF(C.bfid IS NULL, NULL, B.bid), 
    IF(C.bfid IS NULL, NULL, C.cid), 
    IF(C.bfid IS NULL, NULL, C.bfid)
FROM A
LEFT OUTER JOIN B ON A.bfid = B.bid
LEFT OUTER JOIN C ON B.bid = C.bfid
How to write mysql query to achieve the following type of join (Join based on value of a column )

How to write mysql query to achieve the following type of join (Join based on value of a column )


By : Suttipong Chaunket
Date : March 29 2020, 07:55 AM
this one helps. Table users: , You may check the following query:
code :
SELECT
    users.email,
    ub1.user_id,
    ub1.buddy_id,
    ub1.`status`
FROM
    users
JOIN user_buddies ub1 ON users.id = ub1.user_id OR users.id = ub1.buddy_id
WHERE   users.id = 2;
email      user_id     buddy_id     status
p@p.com       1           2         pending (if queried on users.id = 2)
s@s.com       1           2         pending (if queried on users.id = 1)
SELECT
    (
        SELECT
            u.email
        FROM users u
        WHERE   u.id =
        IF (
            users.id = ub1.user_id,
            ub1.buddy_id,
            ub1.user_id
        )
    ) AS buddyEmail,
    ub1.user_id,
    ub1.buddy_id,
    ub1.`status`
FROM
    users
INNER JOIN user_buddies ub1 ON users.id = ub1.user_id
OR users.id = ub1.buddy_id
WHERE   users.id = 2;
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