[日本語]

This is about my experience and also a note that summarizes what I learned from workplace and self-study.

Previously, I was involved in unit test of SQL scripts for securities settlement infrastructure system in Japan. I am beginner engineer and I would like to share my experience here. Please read until the end and please leave a comment.

This time, I would like to share what I was worried about in the PostgreSQL SELECT statement.

As shown below, we recognized that ordinary SELECT statements are used together with the FROM clause.

SELECT *
FROM sample_table;

However, I encountered a SELECT statement that does not have a FROM clause as shown below.

SELECT 'abc'::text AS sample_column;

The result of executing this is as follows:

sample_column
----------
abc

As a result, it succeeded even though there was no FROM clause.

To solve this question, I opened a browser and searched for “SELECT statement without FROM clause” as a keyword. However, no answer was found in the search results.

So I decided to ask my senior at work site. I received a hint from my senior. He said “The FROM clause is omitted sometimes, it is not missing from the SELECT statement”.

I opened the browser again and searched for “SELECT statement with the FROM clause omitted” to find the answer.

SQL: omit the FROM clause

I found that if you do not need to refer to the database table, you can omit the FROM clause in the SELECT statement. For example, when using SELECT instead of a calculator as follows:

SELECT 1*2;

Note that there are DBMS that can omit the FROM clause in SELECT statements and some are cannot.

Thank you for reading.