barcode generator in vb.net code project Defining Assertions in Software

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Defining Assertions
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An assertion is merely a type of CHECK constraint that can be applied to multiple tables. For this reason, an assertion must be created separately from a table definition. Unfortunately, most vendor products, including Oracle 11g, SQL Server 2005, and MySQL 5.0, don t yet support assertions. To create an assertion, use the following syntax: CREATE ASSERTION <constraint name> CHECK <search conditions>
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SQL: A Beginner s Guide
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Creating an assertion is very similar to creating a table CHECK constraint. After the CHECK keyword, you must provide the necessary search condition(s). Now let s take a look at an example. Suppose the CD_TITLES table includes a column for the number of compact discs in stock. You want the total for that table to always be less than the maximum inventory you want to carry. In the following example, I create an assertion that totals the values in the IN_STOCK column and verifies that the total is less than 5000:
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CREATE ASSERTION LIMIT_IN_STOCK CHECK ( ( SELECT SUM (IN_STOCK) FROM CD_TITLES ) < 5000 );
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In this statement, I am using a subquery, (SELECT SUM (IN_STOCK) FROM CD_ TITLES), and comparing it to 5000. The subquery begins with the SELECT keyword, which is used to query data from a table. The SUM function adds the values in the IN_STOCK column, and the FROM keyword specifies which table the column is in. The results of this subquery are then compared (using the less than comparison operator) to 5000. If you try to add a value to the IN_STOCK column that would cause the total to exceed 5000, you will receive an error.
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Creating Domains and Domain Constraints
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The last type of CHECK constraint is the kind that you insert into a domain definition. For the most part, the constraint definition is similar to what you ve seen before, except that you do not tie the constraint to a specific column or table. In fact, domain constraints use the VALUE keyword when referring to the value within a column defined with that particular domain. Let s look at the syntax for creating a domain: CREATE DOMAIN <domain name> [ AS ] <data type> [ DEFAULT <default value> ] [ CONSTRAINT <constraint name> ] CHECK ( <search condition> ) You should already be familiar with most of the elements in this syntax. I discuss data types and default clauses in 3, and the constraint definition is similar to what you ve seen so far in this chapter. In the following example, I create a domain that s based on the INT data type and that requires all values to be between 0 and 30:
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CREATE DOMAIN STOCK_AMOUNT AS INT CONSTRAINT CK_STOCK_AMOUNT CHECK (VALUE BETWEEN 0 AND 30 );
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The only really new item here (other than the CREATE DOMAIN clause) is the keyword VALUE, which, as I said, refers to the value of the column defined with the STOCK_ AMOUNT domain. As a result, if you try to insert a value (into one of those columns) that is not between 0 and 30, you will receive an error.
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4:
Enforcing Data Integrity
Try This 4-2
Adding a CHECK Constraint
In this Try This exercise, which is relatively short, you will be using the ALTER TABLE statement to modify the COMPACT_DISCS table. You will be adding a column to the table and then defining a CHECK constraint that restricts the values that can be entered into the column. The additional column and constraint will have no impact on other tables in the INVENTORY database or on the relationship between tables. You can download the Try_This_04.txtfile, which contains the SQL statements used in this exercise.
Step by Step
1. Open the client application for your RDBMS and connect to the INVENTORY database. 2. You re going to modify the COMPACT_DISCS table by adding the IN_STOCK column.
Enter and execute the following SQL statement:
ALTER TABLE COMPACT_DISCS ADD COLUMN IN_STOCK INT NOT NULL;
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