How do I add a foreign key to an existing SQLite table?

You can't.

Although the SQL-92 syntax to add a foreign key to your table would be as follows:

ALTER TABLE child ADD CONSTRAINT fk_child_parent
                  FOREIGN KEY (parent_id) 
                  REFERENCES parent(id);

SQLite doesn't support the ADD CONSTRAINT variant of the ALTER TABLE command (sqlite.org: SQL Features That SQLite Does Not Implement).

Therefore, the only way to add a foreign key in sqlite 3.6.1 is during CREATE TABLE as follows:

CREATE TABLE child ( 
    id           INTEGER PRIMARY KEY, 
    parent_id    INTEGER, 
    description  TEXT,
    FOREIGN KEY (parent_id) REFERENCES parent(id)
);

Unfortunately you will have to save the existing data to a temporary table, drop the old table, create the new table with the FK constraint, then copy the data back in from the temporary table. (sqlite.org - FAQ: Q11)

As @Daniel Vassallo has said, you can't do it. The code that you have to use is something like this:

Given the table:

CREATE TABLE child( 
id INTEGER PRIMARY KEY, 
parent_id INTEGER, 
description TEXT);

I assume that you want to add the following Foreignk Key:

FOREIGN KEY (parent_id) REFERENCES parent(id);

So I would create a temporary table based on that table, then I would create a new table as the first one but with the Foreign Key and finally I would add the data of the temporary table to it:

CREATE TEMPORARY TABLE temp AS
SELECT 
    id,
    parent_id,
    description
FROM child;

DROP TABLE child;

CREATE TABLE child (
    id INTEGER PRIMARY KEY, 
    parent_id INTEGER, 
    description TEXT,
    FOREIGN KEY(parent_id) REFERENCES parent(id));

INSERT INTO child
 (  id,
    parent_id,
    description)
SELECT
    id,
    parent_id,
    description
FROM temp;

You can try this:

ALTER TABLE [Child] ADD COLUMN column_name INTEGER REFERENCES parent_table_name(column_id);

If you use Db Browser for sqlite ,then it will be easy for you to modify the table. you can add foreign key in existing table without writing a query.

• Open your database in Db browser,
• Just right click on table and click modify,
• At there scroll to foreign key column,
• double click on field which you want to alter,
• Then select table and it's field and click ok.

that's it. You successfully added foreign key in existing table.

Create a foreign key to the existing SQLLite table:

There is no direct way to do that for SQL LITE. Run the below query to recreate STUDENTS table with foreign keys. Run the query after creating initial STUDENTS table and inserting data into the table.

CREATE TABLE    STUDENTS    (       
    STUDENT_ID  INT NOT NULL,   
    FIRST_NAME  VARCHAR(50) NOT NULL,   
    LAST_NAME   VARCHAR(50) NOT NULL,   
    CITY    VARCHAR(50) DEFAULT NULL,   
    BADGE_NO    INT DEFAULT NULL
    PRIMARY KEY(STUDENT_ID) 
);

Insert data into STUDENTS table.

Then Add FOREIGN KEY : making BADGE_NO as the foreign key of same STUDENTS table

BEGIN;
CREATE TABLE STUDENTS_new (
    STUDENT_ID  INT NOT NULL,   
    FIRST_NAME  VARCHAR(50) NOT NULL,   
    LAST_NAME   VARCHAR(50) NOT NULL,   
    CITY    VARCHAR(50) DEFAULT NULL,   
    BADGE_NO    INT DEFAULT NULL,
    PRIMARY KEY(STUDENT_ID) ,
    FOREIGN KEY(BADGE_NO) REFERENCES STUDENTS(STUDENT_ID)   
);
INSERT INTO STUDENTS_new SELECT * FROM STUDENTS;
DROP TABLE STUDENTS;
ALTER TABLE STUDENTS_new RENAME TO STUDENTS;
COMMIT;

we can add the foreign key from any other table as well.

In case somebody else needs info on SQLiteStudio, you can easily do it form it's GUI.

Double-click on the column and double-click foreign key row, then tick foreign key and click configure. You can add the reference column, then click OK in every window.

Finally click on the green tick to commit changes in the structure.

BE AWARE THAT THESE STEPS CREATE SQL SCRIPTS THAT DELETES THE TABLE AND RECREATES IT!!

Backup your data from the database.

Just to visually complete @Gaurav Ganani explanation, which worked fine for me (and i hope it works for those using DB Browser for SQLite), let me paste this image here: