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group by clause Query help

P: n/a
This is my query

select ano,max(date),a_subject from MY_TAB where table_name='xyz' and
ano=877
group by a_subject,ano order by a_subject

ANO max(Date) A_Subject
877 2005-01-20 00:00:00.000 Subject_1
877 1900-01-01 00:00:00.000 Subject_2
877 2004-12-20 00:00:00.000 Subject_3
877 2005-01-19 00:00:00.000 Subject_4
--------------------------------------------------------------------------
When I put the status column in, it fetches all the rows.

select ano,max(date),a_subject,status from MY_TAB where
table_name='xyz' and ano=877 group by a_subject,ano,status order by
a_subject

ANO max(Date) A_Subject Status
877 2005-01-20 00:00:00.000 Subject_1 Not Started
877 1900-01-01 00:00:00.000 Subject_2 Not Started
877 2004-12-20 00:00:00.000 Subject_3 Completed
877 1900-01-01 00:00:00.000 Subject_3 Not Started
877 1900-01-01 00:00:00.000 Subject_4 Not Started
877 2005-01-19 00:00:00.000 Subject_4 Not Started
-----------------------------------------------------------------------
now what i want is

ANO max(Date) A_Subject Status
877 2005-01-20 00:00:00.000 Subject_1 Not Started
877 1900-01-01 00:00:00.000 Subject_2 Not Started
877 2004-12-20 00:00:00.000 Subject_3 Completed
877 2005-01-19 00:00:00.000 Subject_4 Not Started
Thanks a lot for your help.

AJ

Jul 23 '05 #1
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2 Replies


P: n/a
[posted and mailed, please reply in news]

(aj*****@hotmail.com) writes:
select ano,max(date),a_subject from MY_TAB where table_name='xyz' and
ano=877
group by a_subject,ano order by a_subject

ANO max(Date) A_Subject
877 2005-01-20 00:00:00.000 Subject_1
877 1900-01-01 00:00:00.000 Subject_2
877 2004-12-20 00:00:00.000 Subject_3
877 2005-01-19 00:00:00.000 Subject_4
--------------------------------------------------------------------------
When I put the status column in, it fetches all the rows.

select ano,max(date),a_subject,status from MY_TAB where
table_name='xyz' and ano=877 group by a_subject,ano,status order by
a_subject

ANO max(Date) A_Subject Status
877 2005-01-20 00:00:00.000 Subject_1 Not Started
877 1900-01-01 00:00:00.000 Subject_2 Not Started
877 2004-12-20 00:00:00.000 Subject_3 Completed
877 1900-01-01 00:00:00.000 Subject_3 Not Started
877 1900-01-01 00:00:00.000 Subject_4 Not Started
877 2005-01-19 00:00:00.000 Subject_4 Not Started
-----------------------------------------------------------------------
now what i want is

ANO max(Date) A_Subject Status
877 2005-01-20 00:00:00.000 Subject_1 Not Started
877 1900-01-01 00:00:00.000 Subject_2 Not Started
877 2004-12-20 00:00:00.000 Subject_3 Completed
877 2005-01-19 00:00:00.000 Subject_4 Not Started
Thanks a lot for your help.


With the example you have given, you would get away with:

select ano,max(date),a_subject, MIN(status)
from MY_TAB
where table_name='xyz' and ano=877
group by a_subject,ano
order by a_subject

But this does not work well, if you have Status values like "Anticipating"
which represents intermediate stages.

If you are looking for the status on the selected date, then this might be
better:

SELECT a.ano, a.maxdate, a.a_subject, b.status
FROM (select ano, maxdate, max(date), a_subject
from MY_TAB
where table_name='xyz' and ano=877
group by a_subject, ano) AS b
JOIN MY_TAB b ON a.ano = b.ano
AND a.maxdate = b.date
AND a.a_subject = b.a_subject

This assumes that ano/a_subject/date constitutes some form of key in
the table.
--
Erland Sommarskog, SQL Server MVP, es****@sommarskog.se

Books Online for SQL Server SP3 at
http://www.microsoft.com/sql/techinf...2000/books.asp
Jul 23 '05 #2

P: n/a
Here is how a SELECT works in SQL ... at least in theory. Real
products will optimize things when they can.

a) 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.

b) 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 set in the FROM clause.

c) 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
those three items.

d) 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.

e) 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 also give names to expressions in the SELECT
list. 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.

g) The ORDER BY clause is part of a cursor, not a query. The result
set is passed to the cursor, which can only see the names in the SELECT
clause list, and the sorting is done there. The ORDER BY clause cannot
have expression in it, or references to other columns because the
result set has been converted into a sequential file structure and that
is what is being sorted.

As you can see, things happen "all at once" in SQL, not from left to
right as they would in a sequential file/proceudral language model. In
those languages, these two statements produce different results:
READ (a, b, c) FROM File_X;
READ (c, a, b) FROM File_X;

while these two statements return the same data:

SELECT a, b, c FROM Table_X;
SELECT c, a, b FROM Table_X;

Think about what a confused mess this statement is in the SQL model.

SELECT f(c2) AS c1, f(c1) AS c2 FROM Foobar;
That is why such nonsense is illegal syntax.

Jul 23 '05 #3

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