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I can also apply UNIQUE constraints to other columns, but that would not have the same effect as combining two columns into one table constraint, as shown in the following example:
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CREATE TABLE CD_INVENTORY ( ARTIST_NAME VARCHAR(40), CD_NAME VARCHAR(60), COPYRIGHT INT, CONSTRAINT UN_ARTIST_CD UNIQUE ( ARTIST_NAME, CD_NAME ) );
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The ARTIST_NAME column and CD_NAME column must now contain unique combinations of values in order for a row to be added to the CD_INVENTORY table. Until now, I have told you that a UNIQUE constraint prevents duplicate values from being entered into a column or columns defined with that constraint. However, there is one exception to this the null value. A UNIQUE constraint permits multiple null values in a column. As with other columns, null values are permitted by default. You can, however, override the default by using the NOT NULL constraint in conjunction with the UNIQUE constraint. For example, you can add NOT NULL to the CD_NAME column definition:
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CREATE TABLE CD_INVENTORY ( ARTIST_NAME VARCHAR(40), CD_NAME VARCHAR(60) NOT NULL COPYRIGHT INT );
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You can also add NOT NULL to a column definition that s referenced by a table constraint:
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CREATE TABLE CD_INVENTORY ( ARTIST_NAME VARCHAR(40), CD_NAME VARCHAR(60) NOT NULL, COPYRIGHT INT, CONSTRAINT UN_ARTIST_CD UNIQUE (CD_NAME) );
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In each case, both the NOT NULL constraint and the UNIQUE constraint are applied to the CD_NAME column, which means the CD_NAME values must be unique and without null values.
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As I mentioned in the Add UNIQUE Constraints section, a PRIMARY KEY constraint, like the UNIQUE constraint, is a type of SQL unique constraint. Both types of constraints permit only unique values in the specified columns, both types can be applied to one or more columns, and both types can be defined as either column constraints or table constraints. However, PRIMARY KEY constraints have two restrictions that apply only to them:
A column that is defined with a PRIMARY KEY constraint cannot contain null values. It doesn t matter whether or not the column definition specifies NOT NULL the column cannot contain null values because of the PRIMARY KEY constraint. Only one PRIMARY KEY constraint can be defined for each table.
SQL: A Beginner s Guide
The reason for these restrictions is the role that a primary key (unique identifier) plays in a table. As you might recall from 1, each row in a table must be unique. This is important because SQL cannot differentiate between two rows that are completely identical, so you cannot update or delete one duplicate row without doing the same to the other. The primary key for a table is chosen by the database designer from available candidate keys. A candidate key is a set of one or more columns that uniquely identify each row. For example, in Figure 4-4, the only reasonable candidate key in the CD_ARTISTS table is the ARTIST_ID column. Each value in the column will be unique. That way, even if the ARTIST_NAME values and AGENCY values are duplicated, the row is still unique because the ARTIST_ID value is always unique. The uniqueness of a candidate key can be enforced with either a UNIQUE constraint or a PRIMARY KEY constraint. However, each table should include a primary key even if no UNIQUE constraints are defined. This is considered an industry best practice because a primary key cannot accept null values, which makes it the definitive measure by which a row s uniqueness can be ensured. Primary keys are also useful when one table references another through the use of foreign keys. (See the Add FOREIGN KEY Constraints section later in this chapter.) Furthermore, some RDBMSs require the definition of primary keys under certain circumstances, such as when a table column is included in a full text index. To define the primary key, you must use the PRIMARY KEY constraint to specify which column or columns will serve as the table s primary key. The process of defining a PRIMARY
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