You can use a CHECK condition to specify that the values in a column must satisfy some definite criterion.
You can apply a CHECK condition to values in a single column, to specify the rules they should follow. These rules may be rules that data must satisfy in order to be reasonable, or they may be more rigid rules that reflect organization policies and procedures.
CHECK conditions on individual column values are useful when only a restricted range of values are valid for that column. Here are some examples:
You can specify that the entry should match one of a limited number of values. For example, to specify that a city column only contains one of a certain number of allowed cities (say, those cities where the organization has offices), you could use a constraint like the following:
ALTER TABLE office MODIFY city CHECK ( city IN ( 'city_1', 'city_2', 'city_3' ) )
By default, string comparisons are case insensitive unless the database is explicitly created as a case-sensitive database, using the CASE RESPECT option.
You can specify that a date or number falls in a particular range. For example, you may want to require that the start_date column of an employee table must be between the date the organization was formed and the current date, as in the following:
ALTER TABLE employee
MODIFY start_date
CHECK ( start_date BETWEEN '1983/06/27'
AND CURRENT DATE )
You can use several date formats: the YYYY/MM/DD format used in this example has the virtue of always being recognized regardless of the current option settings.
