Restrict Varchar() Column to Specific Values

Restrict varchar() column to specific values?

Have you already looked at adding a check constraint on that column which would restrict values? Something like:

CREATE TABLE SomeTable
(
   Id int NOT NULL,
   Frequency varchar(200),
   CONSTRAINT chk_Frequency CHECK (Frequency IN ('Daily', 'Weekly', 'Monthly', 'Yearly'))
)

Restrict a column of any type to specific values?

You can use this for any data type that supports IN. So it works for strings, numbers, and date/time values.

Restricting a column to accept only 2 values

You need a check constraint.

ALTER TABLE [TableName] ADD CONSTRAINT 
my_constraint CHECK (PatientType = 'Admitted' OR PatientType = 'OPD')

You need to check if it works though in MySQL in particular as of today.

Seems it does not work (or at least it did not a few years ago).

MySQL CHECK Constraint

CHECK constraint in MySQL is not working

MySQL CHECK Constraint Workaround

Not sure if it's fixed now.

If not working, use a trigger instead of a check constraint.

MySQL restricting the value of a MySQL field (allow one specific value and multiple others.)?

If you use null instead of false to indicate that a user is not an owner, then you can use a unique index to provide this restriction because in MySQL unique indexes allow multiple null values. Obviously, you need to make the is_owner field nullable for this to work.

Otherwise, this control is better placed in the application layer, than in the database. In the database you can use a trigger to check this condition and prevent extra owner records to be set by raising an sql error.

In Postgres, how do you restrict possible values for a particular column?

You could add a CHECK CONSTRAINT:

ALTER TABLE distributors 
   ADD CONSTRAINT check_types 
   CHECK (element_type = 'lesson' OR element_type = 'quiz');

Although IMO the cleaner option would be to create an ENUM:

CREATE TYPE element_type AS ENUM ('lesson', 'quiz');

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