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.

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'

Subquery and JOIN

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