Keyword that specifies that the column can contain only unique values. The behavior of the unique table constraint is the same as that for column constraints, with the additional capability to span multiple columns. To define a unique table constraint, use the UNIQUE ( column_name [, ... ] ) syntax.
Keyword that specifies that the column is the primary key for the table. Only one column can be defined as the primary key by using a column definition. To define a table constraint with a multiple-column primary key, use the PRIMARY KEY ( column_name [, ... ] ) syntax.
Identifying a column as the primary key provides metadata about the design of the schema. A primary key implies that other tables can rely on this set of columns as a unique identifier for rows. One primary key can be specified for a table, whether as a column constraint or a table constraint. The primary key constraint should name a set of columns that is different from other sets of columns named by any unique constraint defined for the same table.
Important
Clause that specifies a foreign key constraint, which implies that the column must contain only values that match values in the referenced column of some row of the referenced table. The referenced columns should be the columns of a unique or primary key constraint in the referenced table.
Important
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原文地址：http://blog.csdn.net/tonylee0329/article/details/46773695
