Match column when updating row owb
The syntax is: followed by an identifier followed by the constraint definition.(If you don't specify a constraint name in this way, the system chooses a name for you.) A check constraint can also refer to several columns.Another issue is that you might want to constrain column data with respect to other columns or rows.For example, in a table containing product information, there should be only one row for each product number.Data types are a way to limit the kind of data that can be stored in a table.
But there is no standard data type that accepts only positive numbers.
A uniqueness restriction covering only some rows cannot be written as a unique constraint, but it is possible to enforce such a restriction by creating a unique partial index.
In general, a unique constraint is violated if there is more than one row in the table where the values of all of the columns included in the constraint are equal.
If a user attempts to store data in a column that would violate a constraint, an error is raised.
This applies even if the value came from the default value definition.