Beginning SQL

Chapter 4
Try It Out Adding a CHECK Constraint
1. Using the following SQL, add a CHECK clause to the Age column:
CREATE TABLE NamesAges
(
Name varchar(50),
Age int CHECK (Age >= 0)
);
2. Execute the table creation SQL from Step 1, and then execute the following INSERT INTO
statements:
INSERT INTO NamesAges (Name, Age)
VALUES (‘Jim’, 30);
INSERT INTO NamesAges (Name)
VALUES (‘Jane’);
INSERT INTO NamesAges (Name, Age)
VALUES (‘Will’, -22);
How It Works
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Whenever you add a record to the table, the condition on the Age column must evaluate to true or
unknown for the database to accept the record. If Age is 22, the condition is true and the database inserts
the record. If you try to insert -22, the record violates the CHECK clause and is not inserted. If you insert
no value, the clause evaluates to unknown and is valid.
The first INSERT INTO statement works because the CHECK condition evaluates to true. Indeed, 30 is
greater than or equal to zero.
The second INSERT INTO is successful because the CHECK condition evaluates to unknown, or NULL,
because NULL >= 0 is unknown.
The final INSERT INTO fails because the CHECK condition evaluates to false: -22 >=0 is false because
Age values can’t be negative numbers.
Given that NULL data types are always considered valid, you can prevent them by adding a NOT NULL
constraint. Drop the NamesAges table:
DROP TABLE NamesAges;
Now that you’re through with the Try It Out, consider the following table definition:
CREATE TABLE NamesAges
(
Name varchar(50),
Age int NOT NULL CHECK (Age >= 0)
);