Struggling to figure out how to set these constraints - mysql

Question

I am currently setting up a few different tables for which I have to use certain constraints. I have been getting on okay but I am stuck with the following:
Limiting the 'Country' column to a choice between UK, USA and Australia
Creating the 'ImageFilename' column so that each record must have an extension of .JPG
Is there a specific constraint for these examples or is it a case of thinking outside the box here? I've tried to think of ways to use the current constraints I know but I'm stumped.
Any help would be much appreciated! Thanks in advance.
edit: would it perhaps be a CHECK constraint?

Answer 1

As of MySQL 8.0, MySQL doesn't support CHECK constraints.
Even if MySQL were to add support for CHECK constraints, I wouldn't use that as a solution, because when you eventually add New Zealand and Canada, or if you support .PNG in addition to .JPG, you'd have to redefine the constraints.
For the countries I would create a lookup table, and a foreign key constraint to restrict your country column.
CREATE TABLE Countries ( Country VARCHAR(75) PRIMARY KEY );
ALTER TABLE DifferentTable ADD FOREIGN KEY (Country) REFERENCES Countries(Country);
This allows you to support new countries simply by INSERTing a new country name into the Countries table.
For the image extension, I can think of two alternatives:
Define a trigger that throws a SIGNAL if the filename doesn't match the right extension.
CREATE TRIGGER ImageFilenameIns BEFORE INSERT ON DifferentTable
FOR EACH ROW
BEGIN
IF (SUBSTRING_INDEX(NEW.ImageFilename, -1) <> '.JPG') THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'You must use a .JPG filename';
END IF;
END
CREATE TRIGGER ImageFilenameUpd BEFORE UPDATE ON DifferentTable
FOR EACH ROW
BEGIN
IF (SUBSTRING_INDEX(NEW.ImageFilename, -1) <> '.JPG') THEN
SIGNAL SQLSTATE '45000'
SET MESSAGE_TEXT = 'You must use a .JPG filename';
END IF;
END
Remember to set DELIMITER if you create this trigger using the mysql client or SQL script.
Using the trigger solution requires redefining the trigger if you someday want to support .PNG or other extensions.
Define a generated column to represent the file extension, and make sure that references a lookup table of allowed file extensions.
CREATE TABLE ImageFileExtensions ( Extension VARCHAR(3) PRIMARY KEY );
INSERT INTO ImageFileExtensions (Extension) VALUES ('JPG');
ALTER TABLE DifferentTable
ADD COLUMN ImageFilenameExtension VARCHAR(3) AS (SUBSTRING_INDEX(ImageFilename, -1)) STORED,
ADD FOREIGN KEY(ImageFilenameExtension) REFERENCES ImageFileExtensions(Extension);
With the latter solution, you can add support for a new file extension simply by INSERTing a new extension into the ImageFileExtensions table.

Answer 2

I'm not sure what is that you try to pull off but you can use IF in mysql:
* (for Creating):
CREATE TABLE IF NOT EXISTS
:row(row_id varchar(8) NOT NULL,
:country varchar(25) NOT NULL CHECK (country IN ('USA','UK','Australia')),
PRIMARY KEY (row_id));
Test whether two strings are the same and return "YES" if they are, or "NO" if not (for Selecting):
SELECT IF(STRCMP(":country","UK") = 0, "YES", "NO");
or:
SELECT :row, CASE :Country
WHEN 'USA' THEN 1
WHEN 'UK' THEN 2
WHEN 'Australia' THEN 3
ELSE 'NO'
END AS :row1, :row2, :row3
FROM :Table
but still you can do the same thing with PHP:
$country = $_GET['country'];
switch($country){
case "UK":
case "USA":
case "Australia":
Model::DB->Post($country, "Country");
break;
default:
App::Error->Err("Invalid input - " . $country);
}