ALTER TABLE table_name
ADD CONSTRAINT constraint_name
FOREIGN KEY (column1, column2, ... column_n)
REFERENCES parent_table (column1, column2, ... column_n);
Example
ALTER TABLE products
ADD CONSTRAINT fk_supplier
FOREIGN KEY (supplier_id)
REFERENCES supplier(supplier_id);SELECT c.OWNER, a.TABLE_NAME, a.COLUMN_NAME, a.CONSTRAINT_NAME,
c.R_OWNER AS REF_OWNER, cpk.TABLE_NAME AS REF_TABLE,
cpk.CONSTRAINT_NAME AS REF_PK
FROM ALL_CONS_COLUMNS a
JOIN ALL_CONSTRAINTS c ON a.OWNER = c.OWNER
AND a.CONSTRAINT_NAME = c.CONSTRAINT_NAME
JOIN ALL_CONSTRAINTS cpk ON c.R_OWNER = cpk.OWNER
AND c.R_CONSTRAINT_NAME = cpk.CONSTRAINT_NAME
WHERE c.CONSTRAINT_TYPE = 'R' AND c.TABLE_NAME= 'table_name';SELECT cons.OWNER AS CHILD_OWNER,
cons.TABLE_NAME AS CHILD_TABLE,
cons.CONSTRAINT_NAME AS CONSTAINT_NAME,
cons.CONSTRAINT_TYPE AS CONSTRAINT_TYPE,
col.OWNER AS PARENT_OWNER,
col.TABLE_NAME AS PARENT_TABLE,
col.COLUMN_NAME AS COLUMN_NAME
FROM DBA_CONS_COLUMNS col, DBA_CONSTRAINTS cons
WHERE cons.R_OWNER = col.OWNER
AND cons.R_CONSTRAINT_NAME = col.CONSTRAINT_NAME
AND cons.TABLE_NAME = 'table_name';