Constraints in SQL are the rules applied to the values of individual columns. They add information about how a column can be used after specifying the data type for a column. They can be used to tell the database to reject inserted data that does not adhere to a certain restriction.
Here are some of the constraints that can be set:
PRIMARY KEYcolumns can be used to uniquely identify the row. Attempts to insert a row with an identical value to a row already in the table will result in a constraint violation which will not allow you to insert the new row.
UNIQUEcolumns have a different value for every row. This is similar to
PRIMARY KEYexcept a table can have many different
NOT NULLcolumns must have a value. Attempts to insert a row without a value for a
NOT NULLcolumn will result in a constraint violation and the new row will not be inserted.
DEFAULTcolumns take an additional argument that will be the assumed value for an inserted row if the new row does not specify a value for that column.
Note: There can be only one
PRIMARY KEY column per table, but there can be multiple
The statement below sets constraints on the
idcolumn is the
agecolumn has a