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ORACLE

Stepping through Sub-Queries in Oracle
By: Jagadish Chatarji
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    2006-11-07

    Table of Contents:
  • Stepping through Sub-Queries in Oracle
  • The simplest sub-query in Oracle
  • A sub-query with aggregate functions (or group functions) in Oracle
  • Designing sub-queries to deal with more than one table (or different tables)
  • An example of a nested sub-query (or multi-level sub-query)

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    Stepping through Sub-Queries in Oracle - Designing sub-queries to deal with more than one table (or different tables)


    (Page 4 of 5 )

    Let us consider that I would like to retrieve KING's department name.  All department names are available in the table “dept,” which is quite different from “emp” table.  Let us write the question in a meaningful manner and identify the steps as follows:

    From the above figure, you have two steps to go through with the query.  The following is the order you must follow (based on the above figure):

    • Find KING's department number (1).
    • Based on the value you get, retrieve the department details like dname, loc. etc belonging to that department number (2)

    The following is the statement which retrieves KING's department number:

    SELECT deptno FROM emp WHERE ename = ‘KING’

    To retrieve department details based on a given department number (say 30), the query would be as follows:

    SELECT dname FROM dept

    WHERE deptno = 30

    Just replace the value 30 with the query that gives you KING's department number.  The complete statement would be as follows:

    SQL> SELECT dname FROM dept

    WHERE deptno = (SELECT deptno FROM emp WHERE ename=’KING’);

    Now, let us walk through its execution:

    • The innermost query gets executed first. 
    • In this case, the query “select deptno from emp where ename=’king’” gets executed first.  It retrieves KING's department number.
    • Once the innermost query gets executed, it returns a value to the immediate outer query.  In this case, it is 10.
    • The entire innermost query gets replaced with the new value returned by it.  In this case, the outer-query virtually becomes “select dname from dept where deptno = 10.”
    • And finally, the outer query gets executed, which retrieves KING’s department details

    More Oracle Articles
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       · Hello guys. I started a new series on working with sub-queries in Oracle. Enjoy and...
     

       

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