SQL唯一列(Unique Column)是指在关系数据库中,某一列的值必须是唯一的,不允许重复。唯一列可以用来确保数据的完整性和一致性,避免出现重复数据。 分类: 唯一列属于数据库的约束(Constraint)之一,常见的约束还包括主键(Primary Key)、外键(Foreign Key)和检查约束(Check Constraint)等。 优势: 数据完整性:唯一列可以...
CREATE UNIQUE INDEX for Unique Values If we want to create indexes for unique values in a column, we use theCREATE UNIQUE INDEXconstraint. For example, -- create unique indexCREATEUNIQUEINDEXcollege_indexONColleges(college_code); Run Code Here, the SQL command creates a unique index namedcollege...
UNIQUE constraint ensures that all values in a column are unique.SQL UNIQUE Constraint on CREATE TABLEThe following SQL creates a UNIQUE constraint on the "ID" column when the "Persons" table is created:SQL Server / Oracle / MS Access:...
insert into table_name (column1,column2,column3,...,columnN) values (value1,value2,value3,...,valueN); column1,column2,...,columnN是要插入数据的列的名称。 只要按照与列相同的顺序指定值,就可以指定自己的列顺序。 在插入数据时,也可以将数据插入到特定列中。 五、update和delete语句 1、updat...
②t-sql: alter table Users alter column UserName varchar(50) not null;(以修改列方式) 主键约束:(PK)primary key constraint唯一且不为空; ①设计器:右键对应列标志主键;在键中多一个PK项; ②t-sql:alter table Users add constraintPK_Users_UserIDprimary key (UserID); ...
VALUES TINYINT Var TO VARBINARY,请参阅 BINARY TOP VARCHAR,请参阅 TEXT TRAILING VarP TRANSACTION VARYING TRANSFORM VIEW TRANSLATE WHEN TRANSLATION WHENEVER TRIM WHERE TRUE WITH UNION WORK UNIQUE Xor UNIQUEIDENTIFIER YEAR UNKNOWN YESNO,请参阅 BIT ...
-- Column 'eid' cannot be null 主键不能为空 INSERT INTO emp2 VALUES(NULL,'李逵','男'); # 插入一条数据,主键为 1 -- Duplicate entry '1' for key 'PRIMARY' 主键不能重复 INSERT INTO emp2 VALUES(1,'孙二娘','女'); 3) 哪些字段可以作为主键 ?
A UNIQUE index creates a constraint such that all values in the index must be distinct. An error occurs if you try to add a new row with a key value that matches an existing row. If you specify a prefix value for a column in a UNIQUE index, the column values must be unique within...
UNIQUE DECLARE NATURAL UNKNOWN DEFAULT NCHAR UPDATE DEFERRABLE NEXT UPPER DEFERRED NO USAGE DELETE NONE USER DESC NOT USING DESCRIBE NULL VALUE DESCRIPTOR NULLIF VALUES DIAGNOSTICS NUMERIC VARCHAR DISCONNECT OCTET_LENGTH VARYING DISTINCT OF VIEW
CREATE TABLE 表名 ( 列名1 数据类型 [ NULL | NOT NULL] [ PRIMARY KEY | UNIQUE ] [ FOREIGN...