1. A method comprising: creating a constraint for a database table, wherein the constraint specifies a condition for a first column in the database table and an action, wherein the action specifies whether data that violates the condition is allowed to be stored in the first column; receiving a value and a specification of a second column in the database table from a data source; and if the second column is identical to the first column, the value violates the condition, and the action specifies that data that violates the condition is allowed to be stored, storing the value in a row in the database and marking the row as hidden.