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...
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 ...
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...
Index independent of a constraint Multiple unique nonclustered indexes can be defined on a table. For more information, see CREATE INDEX (Transact-SQL). Indexed view To create an indexed view, a unique clustered index is defined on one or more view columns. The view is executed and the resu...
SQL Server解惑——标识列的限制和跳号现象 1:每个表只能创建一个标识列。 如下测试所示,如果表中有一个标识列,新增一个标识列就会遇到错误“Multiple identity columns specified for table 'TEST'. Only one identity column per table is allowed.“
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# 複製 public System.Collections.Generic.I...
SQL Server解惑——标识列的限制和跳号现象 1:每个表只能创建一个标识列。 如下测试所示,如果表中有一个标识列,新增一个标识列就会遇到错误“Multiple identity columns specified for table 'TEST'. Only one identity column per table is allowed.“
Applies to: SQL Server Azure SQL Database Azure SQL Managed Instance Azure Synapse Analytics Analytics Platform System (PDW) Warehouse in Microsoft Fabric Modifies a table definition by altering, adding, or dropping columns and constraints. ALTER TABLE also reassigns and rebuilds partitions, or disabl...
( 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...