SQL novice looking for a little help

Question

A little SQL help asked for…because my solution works but runs like a pig.
I have two tables with a link many-many association table between them with two FK's defined on the PK's in the tables either side of the join.
CREATE TABLE TABLE1
ID VARCHAR2(20 BYTE) NOT NULL
, NAME VARCHAR2(20 BYTE)
, AGE NUMBER
, ADDR1 VARCHAR2(20 BYTE)
, CONSTRAINT TABLE1_PK PRIMARY KEY
ID
ENABLE
CREATE TABLE TABLE2
ID2 VARCHAR2(20 BYTE) NOT NULL
, TYPE VARCHAR2(20 BYTE)
, DATE_CREATED DATE
, CONSTRAINT TABLE2_PK PRIMARY KEY
ID2
ENABLE
CREATE TABLE LINK_TABLE
ID VARCHAR2(20 BYTE) NOT NULL
, ID2 VARCHAR2(20 BYTE) NOT NULL
, CONSTRAINT LINK_TABLE_PK PRIMARY KEY
ID
, ID2
ENABLE
ALTER TABLE LINK_TABLE
ADD CONSTRAINT LINK_TABLE1_FK1 FOREIGN KEY
ID
REFERENCES TABLE1
ID
ENABLE;
ALTER TABLE LINK_TABLE
ADD CONSTRAINT LINK_TABLE2_FK2 FOREIGN KEY
ID2
REFERENCES TABLE2
ID2
ENABLE;
Populated with the simple data set below that also establishes the many to many links between table1 and table2.
Insert into TABLE1 (ID,NAME,AGE,ADDR1) values ('a','jake',23,'london');
Insert into TABLE1 (ID,NAME,AGE,ADDR1) values ('d','jimmy',26,'lamelane');
Insert into TABLE1 (ID,NAME,AGE,ADDR1) values ('b','jenny',55,'lakeside');
Insert into TABLE1 (ID,NAME,AGE,ADDR1) values ('c','jemima',21,'lothian');
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('z','type1',to_date('10-MAR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('x','type1',to_date('09-MAR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('n','type2',to_date('04-MAR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('d','type1',to_date('03-MAR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('e','type1',to_date('03-MAR-13','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('f','type2',to_date('16-MAR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('s','type4',to_date('04-MAR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('w','type3',to_date('05-APR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('h','type1',to_date('02-APR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('v','type2',to_date('23-APR-12','DD-MON-RR'));
Insert into TABLE2 (ID2,TYPE,DATE_CREATED) values ('k','type3',to_date('30-MAR-12','DD-MON-RR'));
REM INSERTING into LINK_TABLE
SET DEFINE OFF;
Insert into LINK_TABLE (ID,ID2) values ('a','z');
Insert into LINK_TABLE (ID,ID2) values ('a','x');
Insert into LINK_TABLE (ID,ID2) values ('a','f');
Insert into LINK_TABLE (ID,ID2) values ('d','d');
Insert into LINK_TABLE (ID,ID2) values ('d','e');
Insert into LINK_TABLE (ID,ID2) values ('b','w');
Insert into LINK_TABLE (ID,ID2) values ('b','x');
Insert into LINK_TABLE (ID,ID2) values ('b','v');
Insert into LINK_TABLE (ID,ID2) values ('a','n');
Insert into LINK_TABLE (ID,ID2) values ('c','s');
Insert into LINK_TABLE (ID,ID2) values ('c','k');
Insert into LINK_TABLE (ID,ID2) values ('a','s');
Insert into LINK_TABLE (ID,ID2) values ('a','v');
And thus when queried using the join, gives me this data.
select t1.*, t2.* from table1 t1, link_table lt, table2 t2
where lt.id = t1.id
and t2.id2 = lt.id2;
ID NAME AGE ADDR1 ID2 TYPE DATE_CREATED
a jake 23 london f type2 16-MAR-12
a jake 23 london n type2 04-MAR-12
a jake 23 london s type4 04-MAR-12
a jake 23 london v type2 23-APR-12
a jake 23 london x type1 09-MAR-12
a jake 23 london z type1 10-MAR-12
b jenny 55 lakeside v type2 23-APR-12
b jenny 55 lakeside w type3 05-APR-12
b jenny 55 lakeside x type1 09-MAR-12
c jemima 21 lothian k type3 30-MAR-12
c jemima 21 lothian s type4 04-MAR-12
d jimmy 26 lamelane d type1 03-MAR-12
d jimmy 26 lamelane d type1 03-MAR-13
12 rows selected
However, what I need is
for every unique ID in table1, I need the maximum date for in table2 but given that there needs to be a precedence on types in a predefined order, that order being
type4 trumps type2 which trumps type3 which trumps type1.
therefore the query I'm looking for will return the type, type4 and it's max date even if other types exist. If no type4's exist for a link from table1.id in table2 then return type2 and it's max date and so forth.
essentially my query should return
ID     TYPE     MAX_DATE_CREATED
a     type4     04-MAR-12
b     type2     23-APR-12 (since it has no type4 and type2 trumps the other)
c     type4     04-MAR-12
d     type1     03-MAR-13 (the highest precedence is type1 and it's highest date is 03-MAR-13)
Hmmm
I have achieved this using some odd SQL where I use CASE to score the type against the precedence and then sum the total in an aggregate super query but as I say, it runs like a pig, my real dataset is much bigger than this. Hope there's a nice chap/lady out there who rises to this challenge.
TIA
Jenny.

Answer 1

Hi, Jenny,
Welcome to the forum!
Here's one way:
WITH   got_r_num   AS
     SELECT t1.*
     ,      t2.*
     ,      ROW_NUMBER () OVER ( PARTITION BY  t1.id
                                    ORDER BY         CASE    t2.type
                                          WHEN  'type4'  THEN  1
                                          WHEN  'type2'  THEN  2
                                          WHEN  'type3'  THEN  3
                                          WHEN  'type1'  THEN  4
                                      END
                        ,            t2.date_created     DESC
                      )      AS r_num
     FROM   table1          t1
     ,      link_table      lt
     ,      table2          t2
     WHERE  lt.id      = t1.id
     AND    t2.id2      = lt.id2
SELECT     *     -- or whatever columns you want
FROM     got_r_num
WHERE     r_num     = 1
;This is an example of a Top-N Query , where we pick N items (N=1 in this case) from the top of an ordered list (or lists; in this case, we need a separate list for each id.) The analytic ROW_NUMBER function can assign numbers 1, 2, 3, ..., in order, with a separate set of numbers for each id. The tricky part in this problem is getting your trumping order. I used a CASE expresion to map each of the types to a number that reflected the order.
Thanks for posting the CREATE TABLE and INSERT statements; that's very helpful!
Would you like to be even more helpful? Post your best attempt at a query. Even if it performs really poorly, it can help illustrate what you need to do, and maybe give ideas about how to do it. It could be that a small change in what you already have is all that's needed.
The query above will work in Oracle 9.1 or higher. It never hurts to say which version of Oracle you have.