Solved

how does a subquery work

Posted on 2006-10-19
3
915 Views
Last Modified: 2012-08-13
I am searching for an article which tells me how a subquery works in T-SQL. I have a query like:

SELECT colA, colB
FROM TableA a
WHERE
      a.colA = (SELECT max(colA) FROM TableAB ab
                  WHERE a.colC = ab.colC)

how would this query work?
0
Comment
Question by:srinivas_vemla
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
3 Comments
 
LVL 14

Expert Comment

by:Thandava Vallepalli
ID: 17769642
SELECT colA, colB
FROM TableA a
WHERE a.colA =
                     (
                          SELECT max(colA)
                          FROM TableAB ab
                          WHERE a.colC = ab.colC
                     )

This is not subquery.  This is Co-Related Subquery.  
The co-related subquery will be executed for each row in TableA.

Eg:  TableA has 10 rows.  So the subquery will be executed 10 times for each row in TableA.


itsvtk


0
 
LVL 14

Expert Comment

by:Thandava Vallepalli
ID: 17769680
Please find below the description from Books online


Correlated Subqueries
==================
Many queries can be evaluated by executing the subquery once and substituting the resulting value or values into the WHERE clause of the outer query. In queries that include a correlated subquery (also known as a repeating subquery), the subquery depends on the outer query for its values. This means that the subquery is executed repeatedly, once for each row that might be selected by the outer query.

This query finds the names of all authors who earn 100 percent of the shared royalty (royaltyper) on a book.

USE pubs
SELECT au_lname, au_fname
FROM authors
WHERE 100 IN
   (SELECT royaltyper
   FROM titleauthor
   WHERE titleauthor.au_ID = authors.au_id)

Here is the result set:

au_lname                                 au_fname            
---------------------------------------- --------------------
White                                    Johnson              
Green                                    Marjorie            
Carson                                   Cheryl              
Straight                                 Dean                
Locksley                                 Charlene            
Blotchet-Halls                           Reginald            
del Castillo                             Innes                
Panteley                                 Sylvia              
Ringer                                   Albert              

(9 row(s) affected)

Unlike most of the subqueries shown earlier, the subquery in this statement cannot be resolved independently of the main query. It needs a value for authors.au_id, but this value is a variable. It changes as Microsoft® SQL Server™ examines different rows of the authors table.

That is exactly how this query is evaluated: SQL Server considers each row of the authors table for inclusion in the results by substituting the value in each row into the inner query. For example, if SQL Server first examines the row for Cheryl Carson, the variable authors.au_id takes the value 238-95-7766, which SQL Server substitutes into the inner query.

USE pubs
SELECT royaltyper
FROM titleauthor
WHERE au_id = '238-95-7766'

The result is 100, so the outer query evaluates to:

USE pubs
SELECT au_lname, au_fname
FROM authors
WHERE 100 IN (100)

Because this is true, the row for Cheryl Carson is included in the results. Go through the same procedure with the row for Abraham Bennet; you'll see that this row is not included in the results.

itsvtk
0
 
LVL 14

Accepted Solution

by:
Thandava Vallepalli earned 250 total points
ID: 17769722
0

Featured Post

Resolve Critical IT Incidents Fast

If your data, services or processes become compromised, your organization can suffer damage in just minutes and how fast you communicate during a major IT incident is everything. Learn how to immediately identify incidents & best practices to resolve them quickly and effectively.

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

Let's review the features of new SQL Server 2012 (Denali CTP3). It listed as below: PERCENT_RANK(): PERCENT_RANK() function will returns the percentage value of rank of the values among its group. PERCENT_RANK() function value always in be…
I have a large data set and a SSIS package. How can I load this file in multi threading?
Via a live example, show how to extract information from SQL Server on Database, Connection and Server properties
Via a live example, show how to extract insert data into a SQL Server database table using the Import/Export option and Bulk Insert.

738 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question