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Alter table to add foreign key

     
mysql>
mysql> CREATE TABLE T1
    ->       (A INTEGER NOT NULL PRIMARY KEY,
    ->        B INTEGER NOT NULL);
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql>
mysql> CREATE TABLE T2
    ->       (A INTEGER NOT NULL PRIMARY KEY,
    ->        B INTEGER NOT NULL,
    ->        CONSTRAINT C1 CHECK (B > 0),
    ->        CONSTRAINT FK1 FOREIGN KEY (A) REFERENCES T1 (A));
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql> ALTER TABLE T1
    ->    ADD CONSTRAINT FK2 FOREIGN KEY (A) REFERENCES T2 (A);
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>
mysql>
mysql> drop table t1;
ERROR 1217 (23000): Cannot delete or update a parent row: a foreign key constraint fails
mysql> drop table t2;
ERROR 1217 (23000): Cannot delete or update a parent row: a foreign key constraint fails








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