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ORACLE

Inserting Sub-Queries in SELECT Statements in Oracle
By: Jagadish Chatarji
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    2006-11-14


    Table of Contents:
  • Inserting Sub-Queries in SELECT Statements in Oracle
  • Sub-Queries returning single and multiple values in Oracle
  • Sub-Queries as part of the BETWEEN operator in Oracle
  • Derived tables (or inline views) with Sub-Queries in Oracle
  • Sub-Queries with CASE structure in Oracle SELECT statements
  • Sub-Queries as (or part of) columns in Oracle SELECT statements

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    Inserting Sub-Queries in SELECT Statements in Oracle - Sub-Queries with CASE structure in Oracle SELECT statements
    ( Page 5 of 6 )

    Now let us go through an interesting topic on CASE structure.  Let us see how a CASE structure works.  Consider the following statement:

    SELECT
      empno,
      ename,
      CASE job
        WHEN 'SALESMAN' THEN 'SALES'
        WHEN 'MANAGER' THEN 'MGMT'
        ELSE job
      END AS jobfunction
      sal
    FROM emp

    When the above query is executed, it returns four columns (empno, ename, jobfunction, sal). The only eye-catching issue from the above is the following structure:

      CASE job
        WHEN 'SALESMAN' THEN 'SALES'
        WHEN 'MANAGER' THEN 'MGMT'
        ELSE job
      END AS jobfunction

    The above dictates something very similar to the following:

    • If the value of “job” is “salesman” return “sales.”
    • If the above condition fails and if the value of “job” is “manager” return “mgmt.”
    • If both of the above conditions fail then return the same value of “job.”
    • All the values must be returned in a new column with the heading “jobfunction.”

    You need to observe that I specified the column (job) along with CASE. The conditions of WHEN work with the values available within that column. We can even work with different relational (and SQL) operators within the WHEN condition as shown in the following example:

    SELECT
      empno,
      ename,
      CASE 
        WHEN comm IS NULL OR comm=0 THEN '-NA-'
        ELSE TO_CHAR(comm)
      END AS comm,
      sal
    FROM emp

    In the above case, the conditions are directly used within the WHEN statement and you need not specify any column with the CASE.

    Finally, you can even work with sub-queries within the CASE structure as follows:

    SELECT
      empno,
      ename,
      CASE 
        WHEN sal >= (SELECT avg(sal) FROM emp) THEN 'HIGH'
        ELSE 'LOW'
      END AS pay,
      sal
    FROM emp

    The above returns a new column named “pay,” which contains either “HIGH” or “LOW” based on their salary compared to the average salary of all employees.



     
     
    >>> More Oracle Articles          >>> More By Jagadish Chatarji
     

       

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