How to allow a column to be equal to only 1 of 3 values

Question

Hi all while studying for mysql exam i came across this question in a past exam.

Information about the distance travelled so far by each truck is no longer needed. Instead we would like to store information about the present status of a truck. A status can be either Available , Used or Maintained

This status part of the question is where I'm a bit stuck. My only idea to tackle this issue would be through either a where clause or setting a string value between 4 and 9. Another idea i just had as perhaps using a check constraint?

Answer 1

Move the status to a diferent table status(id, name) and store status_id in trucks as a foreign key pointing to one of these

status(1, 'available')
status(2, 'used')
status(3, 'maintained')

Answer 2

The best way to do this would be to create a separate table containing the available status and then implement a foreign key between the trucks relation and the status relation:

CREATE TABLE status (
    id INT NOT NULL PRIMARY KEY,
    description VARCHAR(32)
);

CREATE TABLE trucks (
    id INT NOT NULL PRIMARY KEY,
    status_id INT NOT NULL,
    INDEX idx_status_id (status_id),
    FOREIGN KEY (status_id)
        REFERENCES status(id)
);

Some people would say to use an ENUM type. However, I (and a lot of other DBA's that I have worked with) personally hate the ENUM type. Using an ENUM, you move data from where it belongs to the table metadata, updating the list is a very expensive operation, it is difficult to get all available values from your app, you can't store additional info with it, etc.

Answer 3

You could create ENUM :

An ENUM is a string object with a value chosen from a list of permitted values that are enumerated explicitly in the column specification at table creation time

CREATE TABLE t(
  id INT NOT NULL PRIMARY KEY,
  status ENUM('available','used', 'maintained')
);

db<>fiddle demo