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。
What Are SQL Constraints? SQL constraints are the restrictions on the type of data entered into a database table. Constraints are helpful in enforcing accuracy and reliability in the data through certain conditions. Constraints can be applied both in column level that is it is applied on a sing...
It’s time to understand what this fundamental piece of SQL means and how it can help you accomplish the things you want to accomplish while programming. In thisSQL server trainingyou can learn a lot more than just what constraints are! First, let’s understand exactly what SQL is. SQL st...
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...
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…
小贴士? 在字符串表达式中的字符串需要用引号 " 包含,如果不用引号,SQL会认为是一个属性列的名字,如:col_name = color 表示 col_name和color两个属性一样的行 col_name = "color" 表示 col_name 属性为字符串 "color"的行. 练习 下面再把WHERE语法重复一遍,你可以对照上面的表格,来尝试完成本节课的练习...
With the below result in our example: The ALTER TABLE…DROP CONSTRAINT T-SQL statement can be used easily to drop the previously defined PRIMARY KEY using the name derived from the previous result: 1 2 3 4 ALTER TABLE ConstraintDemo3 DROP CONSTRAINT PK__Constrai__3214EC27E0BEB1C4; If...
The following constraints are commonly used in SQL: NOT NULL- Ensures that a column cannot have a NULL value UNIQUE- Ensures that all values in a column are different PRIMARY KEY- A combination of aNOT NULLandUNIQUE. Uniquely identifies each row in a table ...
cannot be blank. Otherwise, we wouldn’t be able to identify that particular Sale. So generally, there are rules placed on PRIMARY KEY columns in fact tables that ensure the column doesn’t contain duplicates and no row in the column is blank. This is a classic example of a SQL ...