Beginning SQL

If you’re using DB2, the code is as follows:
CREATE TABLE YetAnotherTable
(
Column1 int,
Column2 varchar(20) NOT NULL,
Column3 varchar(12) NOT NULL
);
ALTER TABLE YetAnotherTable
ADD CONSTRAINT MyUniqueConstraint UNIQUE(Column2, Column3);
This constraint is identical to the one created earlier for AnotherTable. Again, you can add more than one
constraint to a table, just as you did with the constraint definition when you defined the table.
Use ALTER TABLE to delete the constraint, and simply drop it as shown below, unless you’re using
MySQL:
ALTER TABLE YetAnotherTable
DROP CONSTRAINT MyUniqueConstraint;
If you’re using MySQL, the code is as follows:
ALTER TABLE YetAnotherTable
DROP INDEX MyUniqueConstraint;
You can also use the preceding code to drop constraints created at the time of a table’s creation, as long
as the constraint has a name.
The table YetAnotherTable can be deleted:
DROP TABLE YetAnotherTable;
CHECK Constraint
Advanced Database Design
The CHECK constraint enables a condition to check the value being entered into a record. If the condition
evaluates to false, the record violates the constraint and isn’t entered into the table. For example,
allowing a column storing an age to contain a negative value doesn’t make sense. After all, how many
people do you know who are minus 35 years old? The CHECK condition can be any valid SQL condition,
similar to those found in a WHERE clause, although SQL can check only the table into which you insert a
record. Note that although MS Access supports the CHECK constraint, its implementation is outside the
scope of this book, and the code examples can’t be executed from the main MS Access query designer.
You can add a CHECK constraint either at the time of a table’s creation or when you alter a table. The following
Try It Out shows you how to use the CHECK constraint.
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