Subquery and JOIN

Converting subqueries into Join functions.

schema:scott

DROP TABLE jobs;
DROP TABLE ranks;
DROP TABLE salary;

 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(40),
   rank VARCHAR(40));
INSERT INTO ranks VALUES ('Lecturer','LECT1');
INSERT INTO ranks VALUES ('Teaching fellow','LECT2');
INSERT INTO ranks VALUES ('Technician','TECH1');
CREATE TABLE salary (
   rank VARCHAR(40),
   payment INTEGER);
INSERT INTO salary VALUES ('LECT1',2000);
INSERT INTO salary VALUES ('LECT2',3000);
INSERT INTO salary VALUES ('TECH1',5000);
INSERT INTO salary VALUES ('TECH2',6000);

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'

JOIN ON functions would also work with

SELECT payment FROM salary JOIN ranks ON (salary.rank = ranks.rank) JOIN jobs ON (ranks.title = jobs.title)

WHERE jobs.employee = 'Andrew Cumming'

{Hacks Ref}