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What's the difference between a primary key and a unique key?

Answer Posted / sunil

HI ALL

IN CASE OF INSERTING REPETED NULL IN UNIQUE KEY COLOUMN ,
IF IT IS COMPOSITE KEY IS POSSIBLE. . . ANY LOGICAL RESON?

Eg:
CREATE TABLE TRY(ROLL NUMBER,LIB_CARD_NO NUMBER,
CONSTRAINT CON_UNQ UNIQUE(ROLL,LIB_CARD_NO));

INSERT INTO TRY VALUES(NULL,NULL);
--> 1 ROW CREATED
INSERT INTO TRY VALUES(NULL,NULL);
--> 1 ROW CREATED -- REPETED NULL ARE ALLOWED!!!!

Is This Answer Correct ?    2 Yes 0 No



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