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CHAPTER 3 De ning Database Objects Using SQL
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NULL | NOT NULL constraint As discussed earlier in this chapter, a speci cation of NULL allows null values in a column, while a speci cation of NOT NULL prohibits null values in the column. Be careful of defaults. If you omit this clause in a Microsoft SQL Server or Sybase database, NOT NULL will be assumed by the DBMS, but if you omit the clause in most other DBMSs, such as Oracle, DB2, MySQL, and PostgreSQL, the behavior is just the opposite: NULL will be assumed. A NOT NULL constraint can also be written as a CHECK constraint (as shown next) that speci es a condition of IS NOT NULL. Here is the SQL syntax and some examples:
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NULL | NOT NULL Examples: DATE_ENROLLED DATE_TERMINATED
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CHECK Constraint A check constraint can be used to enforce any business rule that can be applied to a single column of a table. The condition included in the constraint must always be true whenever the column data in the table is changed or the DBMS rejects the change and displays an error message. An important restriction is that the condition in a column constraint cannot reference any other column. Note in the CUSTOMER_ACCOUNT table example that there are three columns (CUSTOMER_HOLD_INDIC, CREDIT_CARD_ON_FILE_INDIC, and CHILD_RENTAL_ALLOWED_INDIC) that have a CHECK constraint that allows only the values Y and N in the column data. Column constraints may be given an optional name following the CONSTRAINT keyword, which is a good practice because constraint names usually appear in any error message displayed when the constraint is violated. Here is the syntax of a CHECK constraint and an example:
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[CONSTRAINT constraint_name] CHECK (condition) Example: CREDIT_CARD_ON_FILE_INDIC CHAR(1) NOT NULL CHECK (CREDIT_CARD_ON_FILE_INDIC IN 'Y','N'))
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As already mentioned, a NOT NULL clause like the one on the CUSTOMER_ ACCOUNT_ID column can also be written as a CHECK constraint instead. Here is the way the column de nition would look with a check constraint that includes the optional constraint name:
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CUSTOMER_ACCOUNT_ID INTEGER CONSTRAINT CK_CUST_ACCT_ID CHECK (ACCOUNT_ID IS NOT NULL)
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SQL Demysti ed
UNIQUE constraint A unique constraint on a column guarantees unique values for that column within the table, usually with the assistance of an index that is automatically created by the DBMS. Here is the syntax and an example for a column unique constraint:
[CONSTRAINT constraint_name] UNIQUE Example: CUSTOMER_ACCOUNT_ID INTEGER NOT NULL UNIQUE
PRIMARY KEY constraint A primary key constraint on a column declares the column as the primary key of the table, which requires that the column data contain no null values and contain unique values within the table. As with a unique constraint, most DBMSs automatically create an index to assist in checking for unique column data values. Here is the syntax and an example for a column primary key constraint:
[CONSTRAINT constraint_name] PRIMARY KEY Example: CUSTOMER_ACCOUNT_ID INTEGER NOT NULL PRIMARY KEY
Referential (FOREIGN KEY) constraint A referential constraint on a column (sometimes called a foreign key constraint) de nes the relationship between a foreign key and a primary key so that the DBMS may guarantee that the foreign key value, when not null, always references an existing primary key value. The syntax for a column referential constraint is
[CONSTRAINT constraint_name] REFERENCES table_name(column_name) [ON DELETE CASCADE | ON DELETE SET NULL] Example: MPAA_RATING_CODE CHAR(5) NOT NULL REFERENCES MPAA_RATING (MPAA_RATING_CODE)
The optional ON DELETE clause tells the DBMS what to do if the referenced parent table row (the row that contains the primary key) is deleted, with the option to either delete all the rows containing the foreign key value (CASCADE) or to set all the foreign key values to null (SET NULL). Keep in mind that most but not all DBMS implementations support the ON DELETE clause.
Table Constraints
As already mentioned, any column constraint can be written instead as a table constraint, meaning the clause that de nes the constraint appears after all the column de nitions in the CREATE TABLE statement instead of in the midst of a column
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