sys.key_constraints sys.masked_columns sys.記憶體優化表格內部屬性 sys.module_assembly_usages sys.numbered_procedures 系統.編號程序參數 sys.objects sys.parameters sys.partitions sys.periods (系統時期) 系統.計畫指導 sys.procedures sys.sequences sys.server_assembly_modules sys.server_events s...
Primary key Constraints和unique Constraints都需要保证列是唯一的,不能有重复值,但是一张表只能有一个Primary key Constraints,但是可以有多个unique Constraints。还有就是Primary key Constraints是非空的,但是unique Constraints允许为空。不过需要注意的是,因为在SQL Server中可以NULL==NULL,所以unique列只允许只有一个...
sys.key_constraints sys.masked_columns sys.memory_optimized_tables_internal_attributes sys.module_assembly_usages sys.numbered_procedures sys.numbered_procedure_parameters sys.objects sys.parameters sys.partitions sys.periods sys.plan_guides sys.procedures sys.sequences sys.server_assembly_modules sys.serve...
Differences between primary key or unique key constraints and unique indexes It is important to understand that there exists no significant difference between a primary key or unique key constraint and a unique index. To implement the concept of primary and unique key constraints, the database manag...
KEY constraints are defined between the tables, but a foreign key relationship between two tables indicates that the two tables have been optimized to be combined in a query that uses the keys as its criteria. For more information about using FOREIGN KEY constraints with joins, seeJoin ...
SQL Server does not have a predefined limit on either the number of FOREIGN KEY constraints a table can contain (which reference other tables), or the number of FOREIGN KEY constraints owned by other tables that reference a specific table. Nevertheless, the actual number of FOREIGN KEY constrain...
Primary key constraints A table typically has a column or combination of columns that contain values that uniquely identify each row in the table. This column, or columns, is called the primary key (PK) of the table and enforces the entity integrity of the table. Because primary key constrai...
Foreign key constraints (also known as referential constraints or referential integrity constraints) enable you to define required relationships between and within tables.
Consider usingsequencesfor primary key columns to guarantee uniqueness, and avoid the resource overhead that primary key constraints can incur. Setting NOT NULL on Primary Keys When you define a primary key ,Verticaautomatically sets the primary key columns toNOT NULL. For example, when you create...
SQL Server Primary Key Constraints SQL Server主键约束定义唯一约束,同时也会创建一个唯一索引(Unique index),同时这个字段不允许有NULL值存入。 一个表中,只能定义一个主键约束,一旦定义了主键,那它就自动拥有UNIQUE的唯一约束。 下面是在创建表时,定义主键:...