2 .In Existing Table : Example ALTER TABLE UserProfile ADD CONSTRAINT USER_EMAIL_UNIQUE_CONSTRAINT UNIQUE (Email); Drop UNIQUE Key Constraint : Example ALTER TABLE UserProfile DROP CONSTRAINT USER_EMAIL_UNIQUE_CONSTRAINT; Note : In Mysql You have to Use Following Query to Drop the Constraint : ...
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...
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...
处理重复数据:如果业务逻辑允许,可以考虑使用INSERT IGNORE或ON DUPLICATE KEY UPDATE语句来处理重复数据。 示例代码: 代码语言:txt 复制 -- 使用INSERT IGNORE忽略重复数据 INSERT IGNORE INTO users (username, email) VALUES ('john_doe', 'john@example.com'); -- 使用ON DUPLICATE KEY UPDATE更新重复数据 INSE...
This example shows the error produced in such cases: mysql> CREATE TABLE t3 ( -> col1 INT NOT NULL, -> col2 DATE NOT NULL, -> col3 INT NOT NULL, -> col4 INT NOT NULL, -> UNIQUE KEY (col1, col2), -> UNIQUE KEY (col3) -> ) -> PARTITION BY HASH(col1 + col3) ...
SQL Server: CREATE TABLE Customer (SID integer PRIMARY KEY, Last_Name nvarchar(30), First_Name nvarchar(30)); In the examples above, SID column is specified as the primary key. Below is an example of specifying the primary key on a single column usingALTER TABLE. The same SQL statement ...
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: ...
CREATE TABLE users ( id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(255) NOT NULL, email VARCHAR(255) UNIQUE NOT NULL ); 插入数据时,如果违反了唯一约束: 代码语言:txt 复制 INSERT INTO users (name, email) VALUES ('Alice', 'alice@example.com'); INSERT INTO users (name, email) VALUES ...
You can create a unique constraint in SQL Server by using SQL Server Management Studio or Transact-SQL to ensure no duplicate values are entered in specific columns that don't participate in a primary key. Creating a unique constraint automatically creates a corresponding unique index. ...
(2)A department name, where the primary key is the department number Asshown in Example2-1, a unique key constraint exists on the email column of the hr.employeestable. The relevant part of the statement is as follows: ...