Subquery and JOIN

Here you are shown how to convert subqueries into JOIN functions this would be done as using subqueries that contain no aggregate functions unnecessarily causes the response speed of the query to slow down.

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.

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'

Subquery and JOIN

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