I would like to restrict a column value in a SQL table. For example, the column values can only be "car" or "bike" or "van". My question is how do you achieve this in SQL, and is it a good idea to do this on the DB side or should I let the application restrict the input.
I also have the intention to add or remove more values in the future, for example, "truck".
The type of Databases I am using are SQLite and MySQL.
Add a new table containing these means of transport, and make your column a foreign key to that table. New means of transport can be added to the table in future, and your column definition remains the same.
With this construction, I would definitively choose to regulate this at the DB level, rather than that of the application.
For MySQL, you can use the ENUM data type.
column_name ENUM('small', 'medium', 'large')
See MySQL Reference: The ENUM Type
To add to this, I find it's always better to restrict on the DB side AND on the app side. An Enum plus a Select box and you're covered.
Yes, it is recommended to add check constraints. Check constraints are used to ensure the validity of data in a database and to provide data integrity. If they are used at the database level, applications that use the database will not be able to add invalid data or modify valid data so the data becomes invalid, even if the application itself accepts invalid data.
In SQLite:
create table MyTable
(
name string check(name = "car" or name = "bike" or name = "van")
);
In MySQL:
create table MyTable
(
name ENUM('car', 'bike', 'van')
);
You would use a check constraint. In SQL Server it works like this
ALTER TABLE Vehicles
ADD CONSTRAINT chkVehicleType CHECK (VehicleType in ('car','bike','van'));
I'm not sure if this is ANSI standard but I'm certain that MySQL has a similar construct.
If you want to go with DB-side validation, you can use triggers. See this for SQLite, and this detailed how-to for MySQL.
So the question is really whether you should use Database validation or not. If you have multiple clients -- whether they are different programs, or multiple users (with possibly different versions of the program) -- then going the database route is definitely best. The database is (hopefully) centralized, so you can decouple some of the details of validation. In your particular case, you can verify that the value being inserted into the column is contained in a separate table that simply lists valid values.
On the other hand, if you have little experience with databases, plan to target several different databases, and don't have the time to develop expertise, perhaps simple application level validation is the most expedient choice.
To add some beginner level context to the excellent answer of @NGLN above.
First, one needs to check the foreign key constraint is active, otherwise sqlite won't limit to the input to the column to the reference table:
PRAGMA foreign_key;
...which gives a response of 0 or 1, indicating on or off.
To set the foreign key constraint:
PRAGMA foreign_keys = ON;
This needs to be set to ensure that sqlite3 enforces the constraint.
I found it simplest to just set the primary key of the reference table to be the type. In the OP's example:
CREATE TABLE IF NOT EXISTS vehicle_types(
vehicle_type text PRIMARY KEY);
Then, one can insert 'car', 'bike' etc into the vehicle_types table (and more in the future) and reference that table in the foreign key constraint in the child table (the table in which the OP wished to reference the type of vehicle):
CREATE TABLE IF NOT EXISTS ops_original_table(
col_id integer PRIMARY KEY,
...many other columns...
vehicle_type text NOT NULL,
FOREIGN KEY (vehicle_type) REFERENCES vehicle_types(vehicle_type);
Outwith the scope of the OP's question but also take note that when setting up a foreign key constraint thought should be given to what happens to the column in child table (ops_original_table) if a parent table value (vehicle_types) is deleted or updated. See this page for info
