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

I have the following table:

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

How do I add a foreign key constraint on parent_id? Assume foreign keys are enabled.

Most examples assume you're creating the table - I'd like to add the constraint to an existing one.

First add a column in child table Cid as int then alter table with the code below. This way you can add the foreign key Cid as the primary key of parent table and use it as the foreign key in child table ... hope it will help you as it is good for me:

ALTER TABLE [child] 
  ADD CONSTRAINT [CId] 
  FOREIGN KEY ([CId]) 
  REFERENCES [Parent]([CId]) 
  ON DELETE CASCADE ON UPDATE NO ACTION;
GO

Basically you cannot but you can bypass the situation.

The correct way to add the foreign key constraint to an existing table is the following command.

db.execSQL("alter table child add column newCol integer REFERENCES parent(parent_Id)");

Then copy the parent_Id data to the newCol and then delete the Parent_Id column. Hence, no need for temporary table.

You can add the constraint if you alter table and add the column that uses the constraint.

First, create table without the parent_id:

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

Then, alter table:

ALTER TABLE child ADD COLUMN parent_id INTEGER REFERENCES parent(id);

Yes, you can, without adding a new column. You have to be careful to do it correctly in order to avoid corrupting the database, so you should completely back up your database before trying this.

for your specific example:

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

--- create the table we want to reference
create table parent(id integer not null primary key);

--- now we add the foreign key
pragma writable_schema=1;
update SQLITE_MASTER set sql = replace(sql, 'description TEXT)',
    'description TEXT, foreign key (parent_id) references parent(id))'
) where name = 'child' and type = 'table';

--- test the foreign key
pragma foreign_keys=on;
insert into parent values(1);
insert into child values(1, 1, 'hi'); --- works
insert into child values(2, 2, 'bye'); --- fails, foreign key violation

or more generally:

pragma writable_schema=1;

// replace the entire table's SQL definition, where new_sql_definition contains the foreign key clause you want to add
UPDATE SQLITE_MASTER SET SQL = new_sql_definition where name = 'child' and type = 'table';

// alternatively, you might find it easier to use replace, if you can match the exact end of the sql definition
// for example, if the last column was my_last_column integer not null:
UPDATE SQLITE_MASTER SET SQL = replace(sql, 'my_last_column integer not null', 'my_last_column integer not null, foreign key (col1, col2) references other_table(col1, col2)') where name = 'child' and type = 'table';

pragma writable_schema=0;

Either way, you'll probably want to first see what the SQL definition is before you make any changes:

select sql from SQLITE_MASTER where name = 'child' and type = 'table';

If you use the replace() approach, you may find it helpful, before executing, to first test your replace() command by running:

select replace(sql, ...) from SQLITE_MASTER where name = 'child' and type = 'table';

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.

Please check https://www.sqlite.org/lang_altertable.html#otheralter

The only schema altering commands directly supported by SQLite are the "rename table" and "add column" commands shown above. However, applications can make other arbitrary changes to the format of a table using a simple sequence of operations. The steps to make arbitrary changes to the schema design of some table X are as follows:

1. If foreign key constraints are enabled, disable them using PRAGMA foreign_keys=OFF.
2. Start a transaction.
3. Remember the format of all indexes and triggers associated with table X. This information will be needed in step 8 below. One way to do this is to run a query like the following: SELECT type, sql FROM sqlite_master WHERE tbl_name='X'.
4. Use CREATE TABLE to construct a new table "new_X" that is in the desired revised format of table X. Make sure that the name "new_X" does not collide with any existing table name, of course.
5. Transfer content from X into new_X using a statement like: INSERT INTO new_X SELECT ... FROM X.
6. Drop the old table X: DROP TABLE X.
7. Change the name of new_X to X using: ALTER TABLE new_X RENAME TO X.
8. Use CREATE INDEX and CREATE TRIGGER to reconstruct indexes and triggers associated with table X. Perhaps use the old format of the triggers and indexes saved from step 3 above as a guide, making changes as appropriate for the alteration.
9. If any views refer to table X in a way that is affected by the schema change, then drop those views using DROP VIEW and recreate them with whatever changes are necessary to accommodate the schema change using CREATE VIEW.
10. If foreign key constraints were originally enabled then run PRAGMA foreign_key_check to verify that the schema change did not break any foreign key constraints.
11. Commit the transaction started in step 2.
12. If foreign keys constraints were originally enabled, reenable them now.

The procedure above is completely general and will work even if the schema change causes the information stored in the table to change. So the full procedure above is appropriate for dropping a column, changing the order of columns, adding or removing a UNIQUE constraint or PRIMARY KEY, adding CHECK or FOREIGN KEY or NOT NULL constraints, or changing the datatype for a column, for example.

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.

If you are using the Firefox add-on sqlite-manager you can do the following:

Instead of dropping and creating the table again one can just modify it like this.

In the Columns text box, right click on the last column name listed to bring up the context menu and select Edit Column. Note that if the last column in the TABLE definition is the PRIMARY KEY then it will be necessary to first add a new column and then edit the column type of the new column in order to add the FOREIGN KEY definition. Within the Column Type box , append a comma and the

FOREIGN KEY (parent_id) REFERENCES parent(id)

definition after data type. Click on the Change button and then click the Yes button on the Dangerous Operation dialog box.

Reference: Sqlite Manager

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.