using DECODE with FIRST ORDER_BY in an sql query

Question

I have started studying SQL, and while browsing a couple of reading materials I stumbled on this query:

SELECT MAX(SALARY) KEEP (DENSE_RANK FIRST ORDER BY 
    DECODE (SALARY, NULL, NULL, MONTHLY_SAL) NULLS LAST)

I have tried reading up on what this means and have come up with the following (please excuse my long winded and noobish explanations):

1. DECODE (SALARY, NULL, NULL, MONTHLY_SAL) NULLS LAST - will return MONTHLY_SAL unless SALARY is NULL. NULL salaries will also be listed last.

2. MAX(SALARY) - will give the maximum salary that will result from (DENSE_RANK...)

So my questions are these:

1. Will MAX(SALARY) only give one value? or will it return the whole all rows and display the max salary? (ie if I have 2 rows having salaries of 1000 and 2000, the result will be two rows having values of 2000)

2. What is the "total" effect of DENSE_RANK FIRST ORDER BY DECODE (...)? I have only seen examples of ORDER BY and am not sure what will happen if this is coupled with DECODE.

thank you

Answer 1

1) yes, one value

2) when you omit KEEP (DENSE_RANK... you'll get the max salary of all records. With KEEP (DENSE_RANK... you'll get max salary out of all records which have the smallest monthly_sal and a non-null salary. The job of the ´DECODE´-expression is to exclude records with null salary.