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CHAPTER 3 De ning Database Objects Using SQL
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de nition. The main advantage of table constraints over column constraints is that table constraints can reference more than one column. The meaning of each type of constraint has already been covered (in the column constraints topic), so only the general syntax and an example are shown here. The examples all use the CUSTOMER_ACCOUNT table, but some of them have been altered in order to demonstrate key points. CHECK constraint
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[CONSTRAINT constraint_name] CHECK (condition) Example: CONSTRAINT CK_CUSTOMER_DEPOSIT_AMOUNT CHECK (CUSTOMER_DEPOSIT_AMOUNT >= 0 OR CUSTOMER_DEPOSIT_AMOUNT IS NULL)
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This check constraint prevents a negative amount from being stored in the CUSTOMER_DEPOSIT_AMOUNT column. Notice the OR in the expression that also allows null values in the column. If this were not included, any attempt to store a null value would fail because a null value is not greater than or equal to zero. UNIQUE constraint
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[CONSTRAINT constraint_name] UNIQUE (column_name [,column_name...]) Example: CONSTRAINT UK_CUST_ACCT_DATE_ENROLLED UNIQUE (CUSTOMER_ACCOUNT_ID, DATE_ENROLLED)
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This constraint speci es that the combination of CUSTOMER_ACCOUNT_ ID and DATE_ENROLLED be unique among all rows in the CUSTOMER_ ACCOUNT table. However, CUSTOMER_ACCOUNT_ID is already unique all by itself, so adding this constraint doesn t make much sense. I included it here only to illustrate a unique constraint that involves more than one column. PRIMARY KEY constraint
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[CONSTRAINT constraint_name] PRIMARY KEY (column_name [,column_name...]) Example: CONSTRAINT PK_CUSTOMER_ACCOUNT PRIMARY KEY (CUSTOMER_ACCOUNT_ID)
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This is the same as the primary de nition in the CUSTOMER_ACCOUNT table example you have been using in this topic, except that I added a constraint name to it.
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Referential (FOREIGN KEY) constraint
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SQL Demysti ed
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[CONSTRAINT constraint_name] FOREIGN KEY (column_name [,column_name...]) REFERENCES table_name (column_name [,column_name...]) [ON DELETE CASCADE | ON DELETE SET NULL]
Notice that unlike the column constraint form of referential constraint, this one can reference multiple columns. As designed, the CUSTOMER_ ACCOUNT table has no foreign key columns, but let s consider an alternate design. Some database designers frown upon the use of CHECK constraints to control column values because the database design must be changed in order to add or remove values. Suppose, for example, an enhancement to the video store s system requires a new value of E (exempt) for the CREDIT_CARD_ON_FILE_INDIC column. You can change the CHECK constraint to allow the new table, but if you had put all the codes and descriptions in a table (often called a code, reference, or lookup table), all you would have to do is insert a row into the table for the new code value and you d be ready to go. This exibility is exactly why the video store database design contains tables for things like the MPAA_ RATING_CODE when the MPAA changes their rating system, you can simply adjust the data in the code table accordingly. Code tables are also a nice source for populating the pull-down lists of values for application components such as web forms. Assuming that a table called CARD_ON_FILE_TYPE has been created with a primary key of CARD_ON_FILE_CODE, here is the referential constraint that de nes CREDIT_CARD_ON_FILE_INDIC as a foreign key:
CONSTRAINT FK_CARD_ON_FILE_INDIC FOREIGN KEY (CREDIT_CARD_ON_FILE_INDIC) REFERENCES CARD_ON_FILE_TYPE (CARD_ON_FILE_CODE)
As you can see, there isn t always one correct database design but several alternatives from which to choose. Said another way, database design is not an exact science. It is generally considered a best practice to name a foreign key the same as the corresponding primary key column, but as you can see from this example, SQL will let you give them different names if you want or need to.
The CREATE INDEX Statement
The CREATE INDEX statement is a lot simpler than the CREATE TABLE statement that we just explored. Here is the basic syntax:
CHAPTER 3 De ning Database Objects Using SQL
CREATE [UNIQUE] INDEX index_name ON table_name (column_name [ASC | DESC] [, column_name [ASC |DESC]...]);
The optional UNIQUE keyword de nes the index as unique, meaning that no two rows in the table can have the exact same combination of column values. You may be wondering why you would do this when de ning a unique constraint would do the same thing. The answer is that it s a matter of personal style and choice. Some designers don t want the DBMS automatically creating an index for them (as happens with unique constraints), because creating the index themselves gives them more control. Other designers would much rather have the DBMS do the work for them, not only because it s less work, but also because it leads to fewer errors and greater consistency. The optional ASC keyword creates the index in ascending order on the column, while DESC creates the index in descending order on the column. When neither is speci ed, the default is ascending order. An index must have at least one column, but there is no practical upper limit on the number of columns. Indexes are powerful tools because they allow the DBMS to nd data much more quickly, much like using the index in the back of a book to quickly nd a topic of interest. Moreover, indexes on foreign key columns can dramatically improve join performance. However, indexes take storage space and they must be maintained every time a column value referenced by an index is changed, the index must also be changed. The DBMS automatically maintains the index, but the maintenance activity consumes resources on the computer system.
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