How to create a “unique” constraint on a boolean MySQL column?

Question

I would like to add a BOOLEAN column to a MySQL table which will be named is_default. In this column, only one record can have is_default

Answer 1

How are we dealing with this type of problem on DBs?

In some DBMS you can create a partial index.

In PostgreSQL this would look like this:

CREATE UNIQUE INDEX only_one_true 
  ON the_table (is_default)
  WHERE is_default

SQL Server 2008 has a very similar syntax.

On Oracle it's a bit more complicated but doable as well:

CREATE UNIQUE INDEX only_one_true 
  ON the_table (CASE 
                  WHEN is_default = 1 THEN 1
                  ELSE null
                END)

The Oracle solution might work on any DBMS that supports expression for an index definition.