Difference between revisions of "Subquery and JOIN2"

Latest revision as of 14:59, 2 August 2012

In this example you are shown how to convert subqueries containing aggregate functions into JOINs allowing for a more efficient response time from the query.

schema:scott

DROP TABLE orders

 CREATE TABLE orders(
  customer VARCHAR(40),
  whn VARCHAR(40),
  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 a
WHERE a.whn = (
 SELECT MAX(totalitems)
 FROM orders b
 WHERE a.customer = b.customer)

To make this query more efficient a HAVING clause can be used with a self join to replace the subquery.

SELECT a.customer, a.whn, a.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 1: / Line 1: @@
-Converting aggregate subqueries into Join functions.
+In this example you are shown how to convert subqueries containing aggregate functions into
+<code>JOINs</code> allowing for a more efficient response time from the query.
 <div class='ht'>
 <div class=params>schema:scott</div>
 <source lang=sql class='tidy'>DROP TABLE orders</source>
 <source lang=sql class='setup'> CREATE TABLE orders(
-   employee VARCHAR(40),
+   customer VARCHAR(40),
-   whn DATE(YYYY,MM,DD),
+   whn VARCHAR(40),
    totalitems INTEGER );
-INSERT INTO orders VALUES ('Jim',(2006,10,10), 5);
+INSERT INTO orders VALUES ('Jim','2006-10-10', 5);
-INSERT INTO orders VALUES ('Jim',(2006,10,11), 3);
+INSERT INTO orders VALUES ('Jim','2006-10-11', 3);
-INSERT INTO orders VALUES ('Jim',(2006,10,12), 1);
+INSERT INTO orders VALUES ('Jim','2006-10-12', 1);
-INSERT INTO orders VALUES ('Brian',(2006,10,10), 7);
+INSERT INTO orders VALUES ('Brian','2006-10-10', 7);
 </source>
 <div>
@@ Line 16: / Line 17: @@
   FROM orders a
   WHERE a.whn = (
-   SELECT MAX(whn)
+   SELECT MAX(totalitems)
    FROM orders b
    WHERE a.customer = b.customer)
-To make this more efficient a HAVING clause can be used with
+To make this query more efficient a <code>HAVING</code> clause can be used with
-a self join.
+a self join to replace the subquery.
 </div>
-<source lang='sql' class='def'>SELECT a.customer, a.whn, a.totalitems
+<source lang='sql' class='def'>
-FROM orders a JOIN orders b ON (a.customer = b.customer)
+SELECT a.customer, a.whn, a.totalitems
-GROUP BY a.customer, a.whn, a.totalitems
+  FROM orders a JOIN orders b ON (a.customer = b.customer)
-HAVING a.whn = MAX(b.whn)</source>
+  GROUP BY a.customer, a.whn, a.totalitems
+  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"

Latest revision as of 14:59, 2 August 2012

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