Subquery and JOIN

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Revision as of 09:26, 30 July 2012 by Connor (Talk | contribs)

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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'
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