Integrity constraints in SQL are rules enforced on database tables to maintain data accuracy, consistency, and validity, such as ensuring unique primary keys and valid foreign key relationships.
UNIQUE:Unique keyword written with the column name creates a unique key constraint. It ensures that there are no duplicate values in that particular column. Example SQL query tocreate unique constraintswhile creating a table. Query: CREATE TABLE students ( student_ID int UNIQUE, student_Name varch...
This article explains the SQL NOT NULL, Unique and SQL Primary Key constraints in SQL Server with examples. Constraints in SQL Server are predefined rules and restrictions that are enforced in a single column or multiple columns, regarding the values allowed in the columns, to maintain the integr...
SQL, or Structured Query Language, is a powerful tool used for managing and manipulating databases. One of the management tools used to play with and investigate databases is SQL. One feature that ensures accuracy or validity in a database is the implementation of constraints that ensure data is...
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. ...
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 ...
Find the movies not released in the years between 2000 and 2010 Find the first 5 Pixar movies and their release year Stuck? Read this task's Solution. Solve all tasks to continue to the next lesson. Finish above Tasks Next – SQL Lesson 3: Queries with constraints (Pt. 2) Previou...
It will be left behind after you have created the new table with the unique constraint removed. Example Let's look at an example of how to remove a unique constraint from an existing table in SQLite. So say, we already have a suppliers table with the following definition: CREATE TABLE ...
For Example: In the employee table to select the gender of a person, the query would be likeCheck Constraint at column level:CREATE TABLE employee ( id number(5) PRIMARY KEY, name char(20), dept char(10), age number(2), gender char(1) CHECK (gender in ('M','F')), ...
0 - This is a modal window. No compatible source was found for this media. Any constraint that you have defined can be dropped using theALTER TABLEcommand with the DROP CONSTRAINT option. For example, to drop the primary key constraint from the CUSTOMERS table, you can use the following co...