>> Looks like we won't see a "connect-by" chapter in your book;-) <<
I do mention it -- as the work of Satan :)
>> I would mention that there is a striking syntactic and semantic
similarity between "connect-by" and "group-by"... <<
Syntax, maybe if you count using the word "BY" in the construct. But
they are nothing alike.
Here is how a SELECT works in SQL ... at least in theory. Real
products will optimize things when they can.
- Start in the FROM clause and build a working table from all of the
joins, unions, intersections, and whatever other table constructors
are there. The table expression> AS <correlation name> option allows
you give a name to this working table which you then have to use for
the rest of the containing query.
- Go to the WHERE clause and remove rows that do not pass criteria;
that is, that do not test to TRUE (reject UNKNOWN and FALSE). The
WHERE clause is applied to the working in the FROM clause.
- Go to the optional GROUP BY clause, make groups and reduce each
group to a single row, replacing the original working table with the
new grouped table. The rows of a grouped table must be group
characteristics: (1) a grouping column (2) a statistic about the group
(i.e. aggregate functions) (3) a function or (4) an expression made up
of the those three items.
- Go to the optional HAVING clause and apply it against the grouped
working table; if there was no GROUP BY clause, treat the entire table
as one group.
- Go to the SELECT clause and construct the expressions in the list.
This means that the scalar subqueries, function calls and expressions
in the SELECT are done after all the other clauses are done. The AS
operator can give a name to expressions in the SELECT list, too.
These new names come into existence all at once, but after the WHERE
clause, GROUP BY clause and HAVING clause has been executed; you
cannot use them in the SELECT list or the WHERE clause for that
reason.
If there is a SELECT DISTINCT, then redundant duplicate rows are
removed. For purposes of defining a duplicate row, NULLs are treated
as matching (just like in the GROUP BY).
f) Nested query expressions follow the usual scoping rules you would
expect from a block structured language like C, Pascal, Algol, etc.
Namely, the innermost queries can reference columns and tables in the
queries in which they are contained.
The CONNECT BY is a rule for a cursor to traverse a chain.
Received on Sun Feb 02 2003 - 18:54:59 CST