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Design Data Integrity
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CHECK Constraints
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CHECK constraints enforce data integrity by limiting the values that users can enter into a column. CHECK constraints determine whether data is valid by applying a logical expression. You can create CHECK constraints with any logical expression that returns TRUE or FALSE based on the logical operators. For example, if you want to ensure that values in the temperature column are limited to values between 5 and 150, you should use the following logical expression:
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temperature >= -5 AND temperature <= 150
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It is possible to apply multiple CHECK constraints to a single column. It is also possible to apply a single CHECK constraint to several columns within a table. To do this, you must create the CHECK constraint at the table level. CHECK constraints have a problematic relationship with null values because null values evaluate to UNKNOWN. If you have a column that is of type int to which you have applied a CHECK constraint that requires the column value to equal 20, and you then insert the value NULL into the column, the CHECK constraint does not return an error because it can t evaluate the value. To create a CHECK constraint on a table, perform the following steps: 1. Open SSMS, and connect to the database instance that hosts the table that has the column to which you want to apply the CHECK constraint. 2. Expand the Databases folder to locate the database that hosts the table with the column to which you want to apply the CHECK constraint. 3. Expand the appropriate database, and locate the table with the column to which you want to apply the CHECK constraint. 4. Right-click the table and choose Modify. 5. Right-click the column to which you want to apply the CHECK constraint and choose Check Constraints. This opens the Check Constraints dialog box as shown in Figure 8-6. 6. Click Add to add a constraint. 7. In the grid, in the Expression field, type the SQL expression of the CHECK constraint. See the More Info box for more information about creating SQL expressions for CHECK constraints. 8. Click Close.
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Lesson 2: Making Implicit Constraints Explicit
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Figure 8-6
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Adding a Check constraint.
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MORE INFO
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Writing CHECK constraints
For more information about how to write a CHECK constraint, consult the following MSDN article: msdn2.microsoft.com/en-us/library/ms191245(SQL.90,d=ide).aspx.
DEFAULT Definitions
In SQL Server 2005, each column in a record must contain a value, even if that value is NULL. If a column does not allow for null values, you must explicitly specify a value for the column or the database engine returns an error. Because the value NULL might not be desirable and you might need to load a row of data into a table when you do not yet know some column values, you can apply a DEFAULT definition to the column. When you insert a row into a table but don t provide data for the column to which the DEFAULT definition applies, SQL Server automatically inserts the value in the DEFAULT definition.
NOTE
Common default definitions
It is common practice to specify zero as the DEFAULT definition for numeric columns and N/A as the DEFAULT definition for string columns.
To create a DEFAULT definition for a column in a table, perform the following steps: 1. Open SSMS, and connect to the database instance that hosts the table that has the column to which you want to apply the DEFAULT definition.
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Design Data Integrity
2. Expand the Databases folder to locate the database that hosts the table that has the column to which you want to apply the DEFAULT definition. 3. Expand the appropriate database, and locate the table with the column to which you want to apply the DEFAULT definition. 4. Right-click the table and choose Modify. 5. Select the column for which you want to specify a default value. 6. In the Column Properties tab, enter the new default value in the Default Value Or Binding property as shown in Figure 8-7.
Figure 8-7
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