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Join vs Correlated Sub-query

Not always but I observed simple join is much better than Correlated sub-query. So plz avoid to use it, if we have option than ....today i am sharing how a simple join is better than correlated sub query..

correlated sub-query in columns
select  candidate_id,last_name,first_name,client_id, (select name from client_master where client_id=candidate_master.client_id) as cl_name
 from candidate_master


 Output 

(291 row(s) affected)

Stats

Table 'candidate_master'. Scan count 1, logical reads 16, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'client_master'. Scan count 1, logical reads 5, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 107 ms.


 Left outer join 

SELECT        candidate_master.Candidate_ID, candidate_master.last_name, candidate_master.first_name, candidate_master.Client_ID, client_master.name
FROM            candidate_master LEFT OUTER JOIN
                         client_master ON candidate_master.Client_ID = client_master.Client_ID

 Output

(291 row(s) affected)

Stats

Table 'candidate_master'. Scan count 1, logical reads 16, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.
Table 'client_master'. Scan count 1, logical reads 5, physical reads 0, read-ahead reads 0, lob logical reads 0, lob physical reads 0, lob read-ahead reads 0.

 SQL Server Execution Times:
   CPU time = 0 ms,  elapsed time = 70 ms.


Analysis
comparison show how left outer is much better than correlated sub-query. So plz try to avoid this if we have option in simple way.


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Comments

  1. good explanation, how can I get these stats to compare?

    ReplyDelete
  2. Thanks for comments. You can use 'Set Statistics IO On' in any query session.. stats will show on print tab

    ReplyDelete
  3. Howdy! Someone in my Facebook group shared this site with us
    so I came to take a look. I'm definitely enjoying the information. I'm book-marking and will be tweeting this to my followers!
    Outstanding blog and fantastic design.

    ReplyDelete

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