MERGE Constraint Error on self referenced table - sql

Question

MERGE is raising an Error
ORA-02291: integrity constraint violated-parent key not found
on inserting into a self referenced table.
If I change query a little bit with no matter, it will work.
Am I do something wrong.
Execution order:
Preparement stuff
First MERGE query (will not work)
Second MERGE query (will work)
The order of the columns in the MERGE query does not matter. You also can run the second MERGE before first.
If you would like to restart your test, start at DROP TABLE TGR.
First some preparement:
DROP TABLE SRC;
CREATE TABLE SRC (
ID VARCHAR(20) PRIMARY KEY,
PARENT VARCHAR(20)
);
INSERT INTO SRC SELECT '1', null FROM DUAL;
INSERT INTO SRC SELECT '2', '1' FROM DUAL;
DROP TABLE TRG;
CREATE TABLE TRG (
ID VARCHAR(20) PRIMARY KEY,
PARENT VARCHAR(20),
CONSTRAINT FK2 FOREIGN KEY(PARENT) REFERENCES TRG(ID)
);
This will not work:
MERGE INTO TRG t USING(
SELECT PARENT, ID FROM SRC
) s ON (t.ID=s.ID)
WHEN NOT MATCHED THEN INSERT(ID,PARENT) VALUES (s.ID,s.PARENT);
But this will work.
MERGE INTO TRG t USING(
SELECT ID, PARENT FROM SRC
) s ON (t.ID=s.ID)
WHEN NOT MATCHED THEN INSERT(ID,PARENT) VALUES (s.ID,s.PARENT);
If you change the datatype of these columns to NUMBER, it will work.
DROP TABLE SRC;
CREATE TABLE SRC (
ID NUMBER PRIMARY KEY,
PARENT NUMBER
);
INSERT INTO SRC SELECT 1, null FROM DUAL;
INSERT INTO SRC SELECT 2, 1 FROM DUAL;
DROP TABLE TRG;
CREATE TABLE TRG (
ID NUMBER PRIMARY KEY,
PARENT NUMBER,
CONSTRAINT FK2 FOREIGN KEY(PARENT) REFERENCES TRG(ID)
);
MERGE INTO TRG t USING(
SELECT PARENT, ID FROM SRC
) s ON (t.ID=s.ID)
WHEN NOT MATCHED THEN INSERT(ID,PARENT) VALUES (s.ID,s.PARENT);
Edit:
Forgot to tell: I am using 12c