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SQL optimization (inner join or selects)


SQL optimization (inner join or selects)

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SQL optimization (inner join or selects)
Tag : sql , By : Mighty Mac
Date : November 23 2020, 03:01 PM

it should still fix some issue I have a dilema, i had a teacher that thought me basically that inner joins are hell (he reproved me because I missed the delivery of the final proyect by 3 mins...), now i have another that tells me that using just selects is inefficient, so I don't know what is white nor black... could someone enlighten me with their knowledge? , Lets re-write the code so it's easier to understand:
SELECT E.NombreP AS Nombre 
      ,L.Nota 
FROM Lleva L INNER JOIN Estudiante E ON L.CedEstudiante = E.Cedula 
WHERE
    L.SiglaCurso='CI1312';
SELECT L.Nota  
      ,(SELECT E.Nombre 
          FROM Estudiante E 
         WHERE E.Cedula = L.CedEstudiante
       )
FROM Lleva L 
WHERE
    L.SiglaCurso='CI1312'

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MySQL: JOIN syntax + selects within selects = Operand Error


Tag : mysql , By : David Marchant
Date : March 29 2020, 07:55 AM
I wish this helpful for you Well, it looks like after a good night's sleep and some help from a friend, I got this query working. For those that are looking for an answer to a similar question, here is how I got this to work:
select mal,family_name,data,age,status,incidentid
from ( select fn.mal, @fns := @fns + 1 as Sequence
from ( select mal from fnames where mal IS NOT NULL order by rand() limit 100) fn,
(select @fns := 0 ) vars ) as FN
INNER JOIN
(select ln.family_name, @lns := @lns + 1 as Sequence
from ( select family_name from lastnames order by rand() limit 100 ) ln,
(select @lns := 0 ) vars ) as LN
INNER JOIN
(select * from gender2) as Mix0
INNER JOIN
(select * from status) as Mix1
INNER JOIN
(select * from age as Mix2 order by rand() limit 3) as Mix3
INNER JOIN
(select incidentid from incidentid2 as Mix4)
as Mix5 where data='mal' and incidentid='6' and status IN ('inj','ali') and age IN ('NULL','0-17') 
order by rand() limit 100;

SQL JOIN, or two selects?


Tag : mysql , By : RinKaMan
Date : March 29 2020, 07:55 AM
fixed the issue. Will look into that further I'm using a MYSQL database.
SELECT b.DATA 
FROM tbl1 a
LEFT JOIN tbl2 b ON b.ID = a.FID
WHERE a.ID = 2
AAA
BBB
CCC

SQL server join selects slower than join select with local table


Tag : sql , By : user165781
Date : March 29 2020, 07:55 AM
I think the issue was by ths following , You are not giving a lot of information to go by, so my answer is going to be very general.
When SQL Server optimizes a query it tries to find the best posible execution plan. However, for complex queries the number of possible plans is so high that an exhaustive search is not possible.

MySQL : why is left join slower then inner join? Optimization help required


Tag : mysql , By : Ryuken
Date : March 29 2020, 07:55 AM
it helps some times Left join looks for the fields from left + unmatched entries from right, so it has to check every joined field in the right table which might be NULL (if you don't have an index on the fields for that JOIN, it means the query will check the whole right table every time). Inner join looks only for direct matches, so it might not have to go over the whole table to perform a join (Especially if you join on indexed fields).
By the way, if you only want to display the entries mentioned in agent_transition_log, you don't need join at all:
select call_uniqueId, sum(dispo_duration) as DISP, sum(dialing_duration) as DIAL
from agent_transition_log  group by call_uniqueId;
SELECT call_uniqueId, sum(dispo_duration) as DISP, sum(dialing_duration) as DIAL
from agent_transition_log  group by call_uniqueId
UNION
SELECT uniqueid as call_uniqueid, NULL as DISP, NULL as DIAL from closer_log
WHERE uniqueid not in (SELECT call_uniqueid FROM agent_transition_log);

DataFrame join optimization - Broadcast Hash Join


Tag : apache-spark , By : Kilimanjaro
Date : March 29 2020, 07:55 AM
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