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.

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'

SQL HACKS

Hack 10 Converting subqueries into joins

Hack 11 Converting aggregate subqueries into joins

Hack 14 Generate Combinations

Hack 16 Search for a String across columns

Hack 24 Multiply Across a Result Set

Hack 25 Keep a running total

Hack 25.5 Splitting and combining columns

Hack 26 Include the rows your JOIN forgot

Hack 30 Calculate the maximum/minimum of two fields

Hack 33 Get values and subtotals in one shot

Hack 50 Combine tables containing different data

Hack 51/52 Display rows as columns

Hack 55 Import Someone Else's Data

Hack 62 Issue Queries Without Using a Table

Hack 63 Generate rows without tables

Hack 70 Combine your queries

Hack 72 Extract a subset of the results

Hack 78 Break it down by Range

Hack 88 Test two values from a subquery

Hack 98 Find and stop long running queries

Hack 100 Run SQL from a Web Page