Subquery and JOIN
Converting subqueries into Join functions.
DROP TABLE jobs, ranks, 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(20), rank VARCHAR(20)); INSERT INTO ranks VALUES ('Lecturer','LECT1'); INSERT INTO ranks VALUES ('Teaching fellow','LECT2'); INSERT INTO ranks VALUES ('Technician','TECH1'); CREATE TABLE salary ( rank VARCHAR(20), payment INTEGER); INSERT INTO salary VALUES ('LECT1',2000.00); INSERT INTO salary VALUES ('LECT2',3000.00); INSERT INTO salary VALUES ('TECH1',5000.00); INSERT INTO salary VALUES ('TECH2',6000.00 );
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'
SELECT payment FROM salary JOIN ranks ON (salary.rank = ranks.rank) JOIN jobs ON (ranks.title = jobs.title)WHERE jobs.employee = 'Andrew Cumming'