MacLochlainns Weblog

In Alan Beaulieu’s wonderful book Learning SQL: Generate, Manipulate, and Retrieve Data, he uses a SQL 1 to demonstrates a CROSS JOIN in MySQL counting from 1 to 100. My students always find it difficult to read because the subqueries take so much space it makes it difficult to see the process, for example he gives this SQL 1 solution:

SELECT ones.x + tens.x + 1 AS counter
FROM
 (SELECT 0 AS x UNION ALL
  SELECT 1 AS x UNION ALL
  SELECT 2 AS x UNION ALL
  SELECT 3 AS x UNION ALL
  SELECT 4 AS x UNION ALL
  SELECT 5 AS x UNION ALL
  SELECT 6 AS x UNION ALL
  SELECT 7 AS x UNION ALL
  SELECT 8 AS x UNION ALL
  SELECT 9 AS x ) ones CROSS JOIN
 (SELECT 0 AS x UNION ALL
  SELECT 10 AS x UNION ALL
  SELECT 20 AS x UNION ALL
  SELECT 30 AS x UNION ALL
  SELECT 40 AS x UNION ALL
  SELECT 50 AS x UNION ALL
  SELECT 60 AS x UNION ALL
  SELECT 70 AS x UNION ALL
  SELECT 80 AS x UNION ALL
  SELECT 90 AS x ) tens
ORDER BY counter;

While anybody with a command of SQL should be able to see how it works, for those new to SQL it’s difficult. It’s more effective to use a Common Table Expression with the WITH clause because the derived tables become variables in the scope of the WITH clause and the final query works more like a CROSS JOIN between two tables:

WITH ones AS
 (SELECT 0 AS x UNION ALL
  SELECT 1 AS x UNION ALL
  SELECT 2 AS x UNION ALL
  SELECT 3 AS x UNION ALL
  SELECT 4 AS x UNION ALL
  SELECT 5 AS x UNION ALL
  SELECT 6 AS x UNION ALL
  SELECT 7 AS x UNION ALL
  SELECT 8 AS x UNION ALL
  SELECT 9 AS x )
, tens AS
 (SELECT 0 AS x UNION ALL
  SELECT 10 AS x UNION ALL
  SELECT 20 AS x UNION ALL
  SELECT 30 AS x UNION ALL
  SELECT 40 AS x UNION ALL
  SELECT 50 AS x UNION ALL
  SELECT 60 AS x UNION ALL
  SELECT 70 AS x UNION ALL
  SELECT 80 AS x UNION ALL
  SELECT 90 AS x )
SELECT ones.x + tens.x + 1 AS counter
FROM ones CROSS JOIN tens
ORDER BY counter;

As always, I hope this helps somebody trying to sort out the syntax and workflow.

Miraculous events happen on my students’ computers almost weekly. For example, one couldn’t remember their student user’s password, which I set by default. How did they change it? They don’t recall.

Assuming you have sudoer authority on a Linux operating system, you can change a PostgreSQL user’s password as follows:

1. Assume the root user’s role with the following command:

   sudo sh

   sudo sh

2. As the root user, assume the postgres user’s role with the following command:

   su - postgres

   su - postgres

3. As the postgres user, connect to the PostgreSQL database with the following command:

   psql -U postgres

   psql -U postgres

4. Assuming the user’s name is student and you want to set the password to a trivial value like student, use the following command:

   ALTER USER student WITH ENCRYPTED PASSWORD 'student';

   ALTER USER student WITH ENCRYPTED PASSWORD 'student';

As always, I hope this helps somebody trying to sort out the syntax and workflow.