SELECT*FROMusersWHEREemail='alice@example.com'; 1. 执行上述代码后,我们会发现只返回了一条符合条件的数据,即第一条数据。 这是因为在email列上创建了UNIQUE KEY后,MySQL会为该列创建一个索引,可以快速定位到满足条件的数据行。 总结 通过本文,我们了解了MySQL中UNIQUE KEY的作用和用法。 UNIQUE KEY可以用于确...
Unique key is a constraint in SQL Server.This constraint ensures that we cannot enter duplicate values into the columns. It means the data stored in the column is unique among the rows in the table. We can create a unique key on both single and multiple columns. For example, the student...
Example 2: Another example we can take where we can have more than one unique key in just one table: Consider an example where we have aCandidate_Detailtable with the following attributes: Candidate_no:An attribute holding the registration number of the candidate. ...
在MySQL中,`KEY`和`INDEX`实际上是相同的概念,都是用于提高查询性能的数据库对象。它们的主要目的是通过创建索引来加速数据检索。 ### 基础概念 - **索引(Index)**:索引...
(1)A customer phone number, where the primary key is the customernumber (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 state...
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....
key value. In Azure Cosmos DB's API for NoSQL, items are stored as JSON values. These JSON values are case sensitive. When you choose a property as a unique key, you can insert case sensitive values for that property. For example, If you have a unique key defined on the name ...
A uniqueifier (or uniquifier as reported by SQL Server internal tools) is a 4-byte value used to make each key unique in a clustered index that allow duplicate key values. This uniqueness is required because each clustering key must point to exactly one record, without the uniqueifier a ...
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) ...
I have taken another example on column City to display all unique city names stored in the column City. We can see 5 values are stored in the column City for each respective row. Here, Mumbai is saved twice along with the other city names. So when we will use SQL SELECT UNIQUE on th...