Some databases have different rules for primary key constraints.
对于主键约束，有些数据库具有不同的规则。
Some databases have different functionality for primary key constraints.
对于主键约束，有些数据库具有不同的功能。
Consult your database documentation for details about how your database works with primary key constraints.
有关数据库如何使用主键约束的详细信息，请查阅数据库文档。
You can attach unique constraints to columns that allow null values, whereas you can attach primary key constraints only to columns that do not allow null values.
可将唯一约束附加到允许空值的列，但是只能将主键约束附加到不允许空值的列。
Primary key, referential integrity, and check constraints.
主键、引用完整性和检查约束。
You can also use the system catalog views to retrieve information about NOT NULL, unique, primary key, foreign key, and table check constraints. For example.
也可以使用系统编目视图来检索关于NOT NULL、惟一、主键、外键和表检查约束的信息。
SYSCAT.TABLES KEYUNIQUE Number of unique constraints (other than primary key) defined on this table db2 "select tabname, keyunique from syscat.tables where keyunique > 0"
TABLESKEYUNIQUE该表所定义的唯一性（与主键不同）约束的数量db2 "select tabname, keyuniquefrom syscat.tableswhere keyunique > 0"
Constraints such as primary key - foreign key relationships and uniqueness of values are not possible on particular values of an XML document.
不能对XML文档的特定值执行限制，比如主键—外键关系和值的唯一性。
If informational check constraints exist on the primary key column, you should explicitly specify informational check constraints on their corresponding foreign key columns. For example.
如果在主键列上存在信息检查约束，那么应该在它们相应的外键列上指定信息检查约束。
You can attach multiple unique constraints to a table, whereas you can attach only one primary key constraint to a table.
可将多个唯一约束附加到表，但是只能将一个主键约束附加到表。
You can attach multiple unique constraints to a table, whereas you can attach only one primary key constraint to a table.
可将多个唯一约束附加到表，但是只能将一个主键约束附加到表。
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