SQL UNIQUE constraint enables the column to be free from redundant values. It helps restrict the input values to be unique. Thus, by applying a UNIQUE constraint to a column or a set of columns, we ensure that the data contains no Duplicate entries within it. SQL UNIQUE约束使列中没有多余...
To allow naming of a UNIQUE constraint, and for defining a UNIQUE constraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: ALTERTABLEPersonsADDCONSTRAINTuc_PersonIDUNIQUE(P_Id,LastName) To DROP a UNIQUE Constraint To drop a UNIQUE constraint, us...
UNIQUE(ID) ); To name aUNIQUEconstraint, and to define aUNIQUEconstraint on multiple columns, use the following SQL syntax: MySQL / SQL Server / Oracle / MS Access: CREATETABLEPersons ( ID intNOTNULL, LastName varchar(255)NOTNULL,
UNIQUE constraintsConstraints are rules that the SQL Server Database Engine enforces for you. For example, you can use UNIQUE constraints to make sure that no duplicate values are entered in specific columns that don't participate in a primary key. Although both a UNIQUE constraint and a ...
compression; however, compression is not compatible with sparse columns. Lesson 2: Implementing Constraints Quick Check 1. What is the difference between a primary key and a unique constraint? 2. What restrictions does the parent table have when creating a foreign key?
Columns Property Reference Feedback Definition Namespace: Microsoft.SqlServer.TransactSql.ScriptDom Assembly: Microsoft.SqlServer.TransactSql.ScriptDom.dll Package: Microsoft.SqlServer.TransactSql.ScriptDom v161.8901.0 Represents the constraint columns. C# 复制 public System.Collections.Generic.IList...
UNIQUE A constraint that provides entity integrity for a specified column or columns through a unique index. A table can have multiple UNIQUE constraints. UNIQUE can't be specified for CLR table-valued functions. CLUSTERED | NONCLUSTERED Indicate that a clustered or a nonclustered index is cre...
sql server序列 (Introduction to Sequences) 序列是SQL Server 2012中引入的用于密钥生成机制的新对象。 它已在所有版本SQL Server 2012中提供。它是 IDENTITYcolumns feature that has been prevalent in the previous versions of SQL Server. Despite being newly introduced in SQL Server 2012, sequences have ...
UNIQUE A constraint that provides entity integrity for a specified column or columns through a unique index. A table can have multiple UNIQUE constraints. UNIQUE can't be specified for CLR table-valued functions. CLUSTERED | NONCLUSTERED Indicate that a clustered or a nonclustered index is created...
( logical_expression ) ] <column_set_definition> ::= column_set_name XML COLUMN_SET FOR ALL_SPARSE_COLUMNS ::= [ CONSTRAINT constraint_name ] { { PRIMARY KEY | UNIQUE } [ CLUSTERED | NONCLUSTERED ] ( column_name [ ASC | DESC ] [ ,... n ] ) [ WITH FILLFACTOR = fillfactor...