Beginning SQL

Chapter 4
MySQL versions prior to 5.0.2 support the constraints mentioned here but don’t enforce them, which
somewhat reduces their effectiveness. The code described in the following sections works in MySQL, but
when you enter data prohibited by a particular constraint, MySQL won’t reject it as other database systems
would. Discussion of constraints and invalid data as they relate to MySQL is beyond the scope of
this book. Be sure to check the MySQL product documentation for specific information.
NOT NULL Constraint
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Chapter 3 briefly covered the NULL data type. If you recall, NULL is not the same as no data; rather, it represents
unknown data. However, the NULL data type can cause problems with query results. It makes
sorting columns with NULL values difficult, because one NULL is the same as the next, so all the NULLs
appear at the start of any sorted results set. You may also want to ensure that a particular column has a
value — that it’s a compulsory column when inserting a new record. This is where the NOT NULL constraint
comes in. It ensures that the column must have a value or else the record cannot be inserted.
Whether a column in a table can contain NULLs is something that you need to define when you first create
the table. SQL allows NULLs by default, so only when you don’t want NULLs do you need to specify
such in the table creation SQL. Add the constraint to the column definition, immediately after the data
type. For example, the following SQL creates a new table called MyTable and adds three columns, two of
which, Column1 and Column3, specify not to accept NULLs:
CREATE TABLE MyTable
(
Column1 int NOT NULL,
Column2 varchar(20),
Column3 varchar(12) NOT NULL
)
What if you decide later on, after you create a table, that you want to make one or more of the columns
subject to a NOT NULL constraint? Answering this question is a little trickier, and this is another good
reason for planning database table creation in advance of creating the tables. Some DBMSs, such as
Oracle, allow users to modify columns in an existing table with the ALTER TABLE MODIFY statement.
For example, to add a NOT NULL constraint to Column1 in Oracle and MySQL, you would write a statement
similar to the following:
ALTER TABLE MyTable
MODIFY Column2 varchar(20) NOT NULL;
However, many database systems don’t allow this statement, so you must take a different approach:
alter the column definition itself and re-create it with the NOT NULL constraint:
ALTER TABLE MyTable
ALTER COLUMN Column2 varchar(20) NOT NULL;
The ALTER COLUMN statement changes Column2, specifying the same data type but adding the NOT
NULL constraint.