一，Constraint 是表定义的一部分，用于实现数据完整性。
Data Integrity 由三种类型的constraint实现：
Entity Integrity：数据是唯一的。约束： primary key， unique
Domain integrity：data value符合criteria。约束：check，default
Referential integrity：引用的数据必须存在或联动更新。约束：foreign key
二，constraint是database object，所有的contraint name不能重复，必须是唯一的。共有五种类型：primary key，unique，check，default，foreign key，Each contraint has its own row in the sys.objects catalog view.
create table dbo.dt_test ( id int identity not null constraint PK_ID primary key, code int, name varchar(11) ) create table dbo.dt_test_add ( id int identity not null constraint PK_ID check(id>0), code int, name varchar(11) )
Msg 2714, Level 16, State 5, Line 1
There is already an object named ‘PK_ID‘ in the database.
Msg 1750, Level 16, State 0, Line 1
Could not create constraint. See previous errors.
通过sys.objects 查看constraint object的信息，每一个constraint object 都必须依附在parent object，也就是一个base table上。
--C = CHECK constraint --D = DEFAULT (constraint or stand-alone) --F = FOREIGN KEY constraint --PK = PRIMARY KEY constraint --UQ = UNIQUE constraint select NAME,object_id,schema_id,object_name(parent_object_id) as parent_object_name,type,type_desc from sys.objects where type in(‘C‘,‘D‘,‘F‘,‘PK‘,‘UQ‘)
三，跟约束有关的两个column properties 是 Identity和 nullability。
when you use the select into command to make a copy of a table, all column names and data types are copied, as well as Identity and nullability, but constraints are not copied to the new table.
四，约束的定义分为两种: table level 和 column level
参考：CREATE TABLE (Transact-SQL)
https://msdn.microsoft.com/zh-cn/library/ms174979(v=sql.120).aspx
五，约束的默认行为
Primary key 约束和unique 约束都会创建一个unique index，Primary key 约束默认创建clustered unqiue index，Unique 约束默认创建nonclustered unique index，索引类型可以通过关键字 clustered 或nonclustered 来修改。
<column_constraint> ::=
[ CONSTRAINT constraint_name ]
{ { PRIMARY KEY | UNIQUE }
[ CLUSTERED | NONCLUSTERED ]
}
PRIMARY KEY
Is a constraint that enforces entity integrity for a specified column or columns through a unique index. Only one PRIMARY KEY constraint can be created per table.
UNIQUE
Is a constraint that provides entity integrity for a specified column or columns through a unique index. A table can have multiple UNIQUE constraints.
CLUSTERED | NONCLUSTERED
Indicate that a clustered or a nonclustered index is created for the PRIMARY KEY or UNIQUE constraint. PRIMARY KEY constraints default to CLUSTERED, and UNIQUE constraints default to NONCLUSTERED.
In a CREATE TABLE statement, CLUSTERED can be specified for only one constraint. If CLUSTERED is specified for a UNIQUE constraint and a PRIMARY KEY constraint is also specified, the PRIMARY KEY defaults to NONCLUSTERED.
Primary key和Unique的区别是
1.primary key 约束不允许有null值，作为Primary Key 约束的columns 必须是not null；而unique 约束允许有一个null值，在unqieu约束中，null值被认为是相同的，作为unique 约束的columns 可以是null 或 not null。在unique 约束中， (null,A)和(null, null)是不同的，但是(null,null)和(null,null)是相同的。
2.Primary key 约束只能有一个，而unique 约束可以有多个。
3.设计逻辑不同，primary key用于唯一标识表中列，而unique 约束表示数据在table中是唯一的。一个强调的是标识，一个强调的是唯一性。
六，constraint检查的顺序十分重要，一般是按照如下顺序来进行check的。
1, defaut
2, not null
3, check
4,foreigh key
5, unique and primary key
6, trigger fire