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SSQQLL -- PPRRIIMMAARRYY KKEEYY http://www.tutorialspoint.com/sqlite/sqlite_primary_key.htm Copyright © tutorialspoint.com A primary key is a field in a table which uniquely identifies the each rows/records in a database table. Primary keys must contain unique values. A primary key column cannot have NULL values. A table can have only one primary key, which may consist of single or multiple fields. When multiple fields are used as a primary key, they are called a composite key. If a table has a primary key defined on any fields, then you can not have two records having the same value of that fields. Note: You would use these concepts while creating database tables. Create Primary Key: Here is the syntax to define ID attribute as a primary key in a COMPANY table. CREATE TABLE COMPANY( ID INT PRIMARY KEY , NAME TEXT NOT NULL, AGE INT NOT NULL UNIQUE, ADDRESS CHAR (25) , SALARY REAL , ); To create a PRIMARY KEY constraint on the "ID" column when COMPANY table already exists, use the following SQLite syntax: ALTER TABLE COMPANY ADD PRIMARY KEY (ID); For defining a PRIMARY KEY constraint on multiple columns, use the following SQLite syntax: CREATE TABLE COMPANY( ID INT PRIMARY KEY , NAME TEXT NOT NULL, AGE INT NOT NULL UNIQUE, ADDRESS CHAR (25) , SALARY REAL , ); To create a PRIMARY KEY constraint on the "ID" and "NAMES" columns when COMPANY table already exists, use the following SQLite syntax: ALTER TABLE COMPANY ADD CONSTRAINT PK_CUSTID PRIMARY KEY (ID, NAME); Delete Primary Key: You can clear the primary key constraints from the table, Use Syntax: ALTER TABLE COMPANY DROP PRIMARY KEY ; Loading [MathJax]/jax/output/HTML-CSS/fonts/TeX/fontdata.js.

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Referential Integrity in Sqlite
CS 564: Database Management Systems University of Wisconsin - Madison, Fall 2017 Referential Integrity in SQLite Declaring Referential Integrity (Foreign Key) Constraints Foreign key constraints are used to check referential integrity between tables in a database. Consider, for example, the following two tables: create table Residence ( nameVARCHARPRIMARY KEY, capacityINT ); create table Student ( idINTPRIMARY KEY, firstNameVARCHAR, lastNameVARCHAR, residenceVARCHAR ); We can enforce the constraint that a Student’s residence actually exists by making Student.residence a foreign key that refers to Residence.name. SQLite lets you specify this relationship in several different ways: create table Residence ( nameVARCHARPRIMARY KEY, capacityINT ); create table Student ( idINTPRIMARY KEY, firstNameVARCHAR, lastNameVARCHAR, residenceVARCHAR, FOREIGNKEY(residence) REFERENCES Residence(name) ); or create table Residence ( nameVARCHARPRIMARY KEY, capacityINT ); create table Student ( idINTPRIMARY KEY, firstNameVARCHAR, lastNameVARCHAR, residenceVARCHAR REFERENCES Residence(name) ); or create table Residence ( nameVARCHARPRIMARY KEY, 1 capacityINT ); create table Student ( idINTPRIMARY KEY, firstNameVARCHAR, lastNameVARCHAR, residenceVARCHAR REFERENCES Residence-- Implicitly references the primary key of the Residence table. ); All three forms are valid syntax for specifying the same constraint. Constraint Enforcement There are a number of important things about how referential integrity and foreign keys are handled in SQLite: • The attribute(s) referenced by a foreign key constraint (i.e. Residence.name in the example above) must be declared UNIQUE or as the PRIMARY KEY within their table, but this requirement is checked at run-time, not when constraints are declared. For example, if Residence.name had not been declared as the PRIMARY KEY of its table (or as UNIQUE), the FOREIGN KEY declarations above would still be permitted, but inserting into the Student table would always yield an error.
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