Difference between revisions of "Subquery and JOIN"

From SQLZOO
Jump to: navigation, search
Line 1: Line 1:
 
Converting subqueries into Join functions.
 
Converting subqueries into Join functions.
 
<div class='ht'>
 
<div class='ht'>
<div class=params>schema:scott</div>
+
<div class=params>schema:gisq</div>
<source lang=sql class='tidy'>DROP TABLE jobs, ranks, salary</source>
+
<source lang=sql class='tidy'>DROP TABLE jobs</source>
 
<source lang=sql class='setup'> CREATE TABLE jobs(
 
<source lang=sql class='setup'> CREATE TABLE jobs(
 
   employee VARCHAR(40)),
 
   employee VARCHAR(40)),

Revision as of 09:10, 30 July 2012

Converting subqueries into Join functions.

schema:gisq
DROP TABLE jobs
 CREATE TABLE jobs(
  employee VARCHAR(40)),
  title VARCHAR(40));
INSERT INTO jobs VALUES ('Gordon Russell','Lecturer');
INSERT INTO jobs VALUES ('Andrew Cumming','Teaching fellow');
INSERT INTO jobs VALUES ('Jim Smith','Technician');
CREATE TABLE ranks (
   title VARCHAR(20),
   rank VARCHAR(20));
INSERT INTO ranks VALUES ('Lecturer','LECT1');
INSERT INTO ranks VALUES ('Teaching fellow','LECT2');
INSERT INTO ranks VALUES ('Technician','TECH1');
CREATE TABLE salary (
   rank VARCHAR(20),
   payment INTEGER);
INSERT INTO salary VALUES ('LECT1',2000.00);
INSERT INTO salary VALUES ('LECT2',3000.00);
INSERT INTO salary VALUES ('TECH1',5000.00);
INSERT INTO salary VALUES ('TECH2',6000.00 );
SELECT payment FROM salary WHERE rank =
(SELECT rank FROM ranks WHERE title =
(SELECT title FROM jobs 
 WHERE employee = 'Andrew Cumming'))

This subquery would be more efficient if it was changed to a JOIN as there are no aggregate functions in the queries.

    Use the following steps to change a subquery into a JOIN
  • Mark all columns with the table name they came from
  • If you use the same table in two different FROM clauses, use aliases
  • Move all FROM statements together to form a single FROM
  • Delete all occurrences of (SELECT
  • Substitute WHERE for AND after the first occurence of WHERE
SELECT payment FROM salary, ranks, jobs
WHERE salary.rank = ranks.rank
AND ranks.title = jobs.title
AND jobs.employee = 'Andrew Cumming'

{Hacks Ref}

Personal tools
Namespaces

Variants
Actions
Reference
Toolbox
Google AdSense