CHECK Constraints
CHECK constraints enforce domain integrity by limiting the values that are accepted by a column. They are similar to FOREIGN KEY constraints in that they control the values that are placed in a column. The difference is in how they determine which values are valid: FOREIGN KEY constraints get the list of valid values from another table, and CHECK constraints determine the valid values from a logical expression that is not based on data in another column. For example, it is possible to limit the range of values for a salary column by creating a CHECK constraint that allows only data that ranges from $15,000 through $100,000. This prevents salaries from being entered beyond the normal salary range.
You can create a CHECK constraint with any logical (Boolean) expression that returns TRUE or FALSE based on the logical operators. For the previous example, the logical expression is:
salary >= 15000 AND salary <= 100000
It is possible to apply multiple CHECK constraints to a single column. These are evaluated in the order in which created. It is also possible to apply a single CHECK constraint to multiple columns by creating it at the table level. For example, a multiple-column CHECK constraint can be used to confirm that any row with a country column value of USA also has a two-character value in the state column. This allows multiple conditions to be checked in one place.