Defining Table Constraints
Constraints provide a means to enforce data integrity. In addition to NULL/NOT NULL, discussed earlier in this chapter, SQL Server provides five constraint types: PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK, and DEFAULT. These constraints help further define the type of data you can store in tables.
Constraints are covered in detail in Chapter 23, “Implementing Data Integrity.” This chapter introduces the basic means for adding constraints to a table. You can add constraints at the time of table creation, or you can add them after a table has been created by using the ALTER TABLE statement.
Listing 21.9 shows a CREATE TABLE statement that has an example of each one of the five constraint types listed. The PRIMARY KEY
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