It's not allowed per the SQL-92 standard, but permitted in MySQL. From the MySQL manual:
MySQL permits you to create a column of type
CHAR(0)
. This is useful primarily when you have to be compliant with old applications that depend on the existence of a column but that do not actually use its value.CHAR(0)
is also quite nice when you need a column that can take only two values: A column that is defined asCHAR(0)
NULL occupies only one bit and can take only the valuesNULL
and''
(the empty string).