In this blog post, I will visit a couple of handy string functions for extracting a specified number of characters from a string of text. Yet, with both functions, you can start from a certain position. Either left (beginning) or right (the end). Let’s see them in action with some examples…
- Xubuntu Linux 18.04.2 LTS (Bionic Beaver)
- PostgreSQL 11.2
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For these examples, I will use the ‘actor’ table from the mock DVD Rental practice database ported to Postgres, based on the open source SQL Sakila database.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 | dvdrental=> \d actor; Table "public.actor" Column | Type | Collation | Nullable | Default -------------+-----------------------------+-----------+----------+----------------------------------------- actor_id | integer | | not null | nextval('actor_actor_id_seq'::regclass) first_name | character varying(45) | | not null | last_name | character varying(45) | | not null | last_update | timestamp without time zone | | not null | now() Indexes: "actor_pkey" PRIMARY KEY, btree (actor_id) "idx_actor_last_name" btree (last_name) Referenced by: TABLE "film_actor" CONSTRAINT "film_actor_actor_id_fkey" FOREIGN KEY (actor_id) REFERENCES actor(actor_id) ON UPDATE CASCADE ON DELETE RESTRICT Triggers: last_updated BEFORE UPDATE ON actor FOR EACH ROW EXECUTE PROCEDURE last_updated() |
Let’s dive right in with the left() function first:
1 2 3 4 5 6 7 8 9 | dvdrental=> SELECT first_name, left(first_name, 2) dvdrental-> FROM actor dvdrental-> WHERE first_name LIKE 'V%'; first_name | left ------------+------ Vivien | Vi Val | Va Vivien | Vi (3 rows) |
From the query results above, you can see that left() returned 2 characters from the left of the supplied string argument (or the first count of characters where count is the integer argument), in this case, the ‘first_name’ column. I only limited the returned number of rows via the WHERE clause with a predicate check for those names that start with ‘V’, for better on-screen visibility. The filter has nothing to do with the left() function.
The official documentation (see link below) defines the left() function as:
Return first n characters in the string. When n is negative, return all but last |n| characters.
As intelligent as you all are, I am sure you know what the right() function does then, right?
1 2 3 4 5 6 7 8 9 | dvdrental=> SELECT first_name, right(first_name, 2) dvdrental-> FROM actor dvdrental-> WHERE first_name LIKE 'V%'; first_name | right ------------+------- Vivien | en Val | al Vivien | en (3 rows) |
You are correct!
The same is true for the right() function with the exception that right() extracts characters from the right, or end of the string.
From the docs, right() is defined as:
Return last n characters in the string. When n is negative, return all but first |n| characters.
Have a look at the left() and right() string functions next time you need to remove a certain number of characters from a string at either its beginning or end. Note there is different functionality when the n argument is negative, and that is perhaps another post for another day…
Like what you have read? See anything incorrect? Please comment below and thanks for reading!!!
Explore the official PostgreSQL 11 On-line Documentation for more information.
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Josh Otwell has a passion to study and grow as a SQL Developer and blogger. Other favorite activities find him with his nose buried in a good book, article, or the Linux command line. Among those, he shares a love of tabletop RPG games, reading fantasy novels, and spending time with his wife and two daughters.
Disclaimer: The examples presented in this post are hypothetical ideas of how to achieve similar types of results. They are not the utmost best solution(s). The majority, if not all, of the examples provided, is performed on a personal development/learning workstation-environment and should not be considered production quality or ready. Your particular goals and needs may vary. Use those practices that best benefit your needs and goals. Opinions are my own.
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