Difference between revisions of "Subquery and JOIN2"

Revision as of 09:42, 30 July 2012

Converting aggregate subqueries into Join functions.

schema:scott

DROP TABLE orders

 CREATE TABLE orders(
  employee VARCHAR(40)),
  whn DATE(YYYY,MM,DD),
  totalitems INTEGER));
INSERT INTO orders VALUES ('Jim',(2006,10,10), 5);
INSERT INTO orders VALUES ('Jim',(2006,10,11), 3);
INSERT INTO orders VALUES ('Jim',(2006,10,12), 1);
INSERT INTO orders VALUES ('Brian',(2006,10,10), 7);

SELECT customer, whn, totalitems
FROM orders 01
WHERE 01.whn = (
 SELECT MAX(whn)
 FROM orders 02
 WHERE 01.customer = 02.customer)

To make this more efficient a HAVING clause can be used with a self join.

SELECT 01.customer, 01.whn, 01.totalitems
FROM orders a JOIN orders b ON (a.customer = b.customer)
GROUP BY a.customer, a.whn, a.totalitems
HAVING a.whn = MAX(b.whn)

@@ Line 23: / Line 23: @@
 </div>
 <source lang='sql' class='def'>SELECT 01.customer, 01.whn, 01.totalitems
-FROM orders 01 JOIN order 02 ON (01.customer = 02.customer)
+FROM orders a JOIN orders b ON (a.customer = b.customer)
-GROUP BY 01.customer, 01.whn, 01.totalitems
+GROUP BY a.customer, a.whn, a.totalitems
-HAVING 01.whn = MAX(02.whn)</source>
+HAVING a.whn = MAX(b.whn)</source>
 <div class="ecomm e-mysql" style="display: none"></div>
 </div>
 {{Hacks Ref}}

Difference between revisions of "Subquery and JOIN2"

Revision as of 09:42, 30 July 2012

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