Throughout this article, we’ll use this example database and show several ways to enforce data integrity. We’ll use PostgreSQL syntax in our queries. However, the concepts easily translate to other flavors of SQL. PRIMARY KEY Constraint The university aims to identify each student uniquely. ...
为了更精确的查询出特定数据,我们需要学习一个新的SQL语法:SELECT查询的 WHERE 子句. 一个查询的 WHERE子句用来描述哪些行应该进入结果,具体就是通过 condition条件 限定这些行的属性满足某些具体条件。比如:WHERE 体重大于 10KG的狗。你可以把 WHERE想象成一个 筛子,每一个特定的筛子都可以筛下某些豆子。
专注数据分析30年 sql数据分析社群 xuesql.cn 我们已经学会了WHERE 语句来筛选数字类型的属性,如果属性是字符串, 我们会用到字符串相关的一些操作符号,其中 LIKE(模糊查询) 和 %(通配符) 是新增的两个. 下面这个表格对字符串操作符有详细的描述: Operator(操作符) Condition(解释) Example(例子) = Case sensitiv...
SQL | Constraints: In this tutorial, we are going to learn about the various constraints in SQL with its usages, syntaxes and query examples.
When designing an SQL database, there may be cases where you want to impose restrictions on what data can be added to certain columns in a table. SQL makes t…
For example, if two users use the same mobile number, as the mobile number is a unique identifier, this constraint will make sure that it is not repeated. Major importance of SQL Constraints: It creates a relationship between the tables in a database. Maintains consistency in the tables....
By default, SQL Server allows storing NULL values in columns. These NULL values do not represent valid data. For example, every employee in an organization must have an Emp ID, first name, gender and address. Therefore, you can specify a column with NOT NULL constraints to always ensure val...
SQL query to create default constraints while creating a table. Query: CREATE TABLE Students( Student_ID int NOT NULL, Student_Name varchar(255) NOT NULL, Class_Name varchar(255) DEFAULT 'IV' ); Output: In the above example, the Class_Name column in the student’s table will default the...
For example, to drop the primary key constraint in the USERS table, you can use the this command: ALTER TABLE USERS DROP CONSTRAINT USERS_PK; So, now you have a general idea of how some common constraints are used and how they can help you customize your database. Constraints can be sp...
Also, you cannot enter NULL value in a primary key column.The following SQL statement creates a table named persons and specifies the id column as the primary key. That means this field does not allow NULL or duplicate values.ExampleTry this code » CREATE TABLE persons ( id INT NOT ...