Question

An example from my Oracle SQL Certified Expert Exam study guide has me puzzled.

The SQL statement:

INSERT
  WHEN (BOSS_SALARY-EMPLOYEE_SALARY < 10000) THEN
    INTO SALARY_CHART (EMP_TITLE, SUPERIOR, EMP_INCOME, SUP_INCOME)
    VALUES (EMPLOYEE, BOSS, EMPLOYEE_SALARY, BOSS_SALARY)
SELECT A.POSITION EMPLOYEE
  , B.POSITION BOSS
  , A.MAX_SALARY EMPLOYEE_SALARY
  , B.MAX_SALARY BOSS_SALARY
FROM POSITIONS A
  JOIN POSITIONS B ON A.REPORTS_TO_POSITION_ID = B.POSITION_ID

The explanation from the book:

Note that this version has done more
than is required, and applies column
aliases to each column in the
subquery, then references those column
aliases from the WHEN and VALUES
clauses. We only needed column
aliases on A.POSITION and B.POSITION
in lines 5 and 6, so we can reference the column aliases in line 4.

My source of confusion:

Don’t we also need the aliases on the two MAX_SALARY columns selected in lines 7 and 8, in order to distinguish them in line 4? The explanation seems to suggest the EMPLOYEE_SALARY and BOSS_SALARY aliases are extraneous…

Can someone please clarify this?

Answer 1

I’m guessing they meant they only need it on two columns (eg A.POSITION and A.MAX_SALARY) to disambiguate for the parser. For example, this would work:

INSERT
  WHEN (MAX_SALARY-EMPLOYEE_SALARY < 10000) THEN
    INTO SALARY_CHART (EMP_TITLE, SUPERIOR, EMP_INCOME, SUP_INCOME)
    VALUES (EMPLOYEE, POSITION, EMPLOYEE_SALARY, MAX_SALARY)
SELECT A.POSITION EMPLOYEE
  , B.POSITION 
  , A.MAX_SALARY EMPLOYEE_SALARY
  , B.MAX_SALARY 
FROM POSITIONS A
  JOIN POSITIONS B ON A.REPORTS_TO_POSITION_ID = B.POSITION_ID