SQL UNIQUE 约束 UNIQUE(唯一性) 约束唯一标识数据库表中的每条记录。 UNIQUE(唯一性) 和 PRIMARY KEY(主键) 约束均为列或列集合提供了唯一性的保证。 PRIMARY KEY(主键) 约束拥有自动定义的 UNIQUE(唯一性) 约束。 一)CREATE TABLE 时的 SQL UNIQUE 约束 1、创建表时,定义单个列的 U
而且对表索引时不会存储NULL值的，所以如果索引的字段可以为NULL，索引的效率会下降很多。
The following table cannot be partitioned at all, because there is no way to include in a partitioning key any columns that belong to both unique keys: CREATE TABLE t4 ( col1 INT NOT NULL, col2 INT NOT NULL, col3 INT NOT NULL, col4 INT NOT NULL, UNIQUE KEY (col1, col3), ...
APRIMARY KEYconstraint automatically has aUNIQUEconstraint. However, you can have manyUNIQUEconstraints per table, but only onePRIMARY KEYconstraint per table. SQL UNIQUE Constraint on CREATE TABLE The following SQL creates aUNIQUEconstraint on the "ID" column when the "Persons" table is created: ...
非空约束(NOT NULL):非空约束要求某列的数据不能为空 唯一约束(UNIQUE):唯一约束要求某列的数据必须唯一,但允许为空。唯一约束可以作用于单列或多列(全部列相同视为不唯一) 检查约束(CHECK):检查约束用于限制某列的取值范围或格式 主键约束(PRIMARY KEY):主键约束要求主键列的数据必须唯一且不能为空。一个表中...
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A Primary Key Constraint will not allow a duplicate value or NULL in a column in order to enforce referential integrity. Create a table with a Constraint called PK_EmpNum on the EmpNum field. CREATE TABLE [dbo].[Employees_6]( [EmpNum] [int] NOT NULL, ...
DETAIL: Key (x, (y IS NULL))=(2, t) already exists.INSERT INTO nulltest2 VALUES (2, 3); INSERT INTO nulltest2 VALUES (2, 3); Thei_nulltest2index allows only oneynull value for eachxvalue. This can actually be useful in certain data models. This illustrates how expression and...
We can define multiple Unique keys on a table where one or more columns combine to make a Unique key. According to ANSI, we can use various NULL values, but we can add only one NULL value in the SQL server. Multiple NULLs cannot be inserted with the Unique key, but you can create ...
- A primary key can’t be created on a null able column wherein a unique key can be created on a null able column. - There can be only one primary key on a table wherein a table can have multiple unique key defined on it. ...