Oracle SQL - Constraints

jiselectric·2020년 9월 23일

Database and Application

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SQL Constraints

SQL Constraints are rules used to limit the type of data that can go into a table, to maintain the accuracy and integrity of the data inside table.


CREATE TABLE table_name (
    column1 datatype constraint, 
    column2 datatype constraint, 
    column3 datatype constraint

List of Commonly Used SQL Constraints

  • NOT NULL : the data of the column must have a value (can not have NULL value)
  • UNIQUE : ensures that all values in a column are different
  • PRIMARY KEY : ensures to uniquely identify each row in table (UNIQUE + NOT NULL)
  • FOREIGN KEY : key used to link two tables together, commonly dependant on Primary Key
  • CHECK : ensures that all values in a column satisfies a specific condition (boolean expression whether the condition is satisfied)
  • DEFAULT : when no value is assigned to a column, assigns specific default value
  • INDEX : used to quickly retrieve data from the table


create a table students containing student's basic information

CREATE TABLE students (
    student_id NUMBER PRIMARY KEY,
    student_firstName VARCHAR2(50) NOT NULL,
    student_lastName VARCHAR2(50) NOT NULL,
    student_major VARCHAR2(50) NOT NULL,
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