SQL Unique Key
1.What will be the output of the given code?
CREATE TABLE emails (
email VARCHAR(100) UNIQUE
);
INSERT INTO emails VALUES ('[email protected]');
INSERT INTO emails VALUES ('[email protected]');
-
A. Both inserted
-
B. Error: Duplicate UNIQUE
-
C. Replaces old row
-
D. Inserts NULL
Error: Duplicate UNIQUE
Error: Duplicate UNIQUE
2.Predict the output of the below code?
CREATE TABLE contacts (
phone VARCHAR(10) UNIQUE
);
INSERT INTO contacts VALUES (NULL);
INSERT INTO contacts VALUES (NULL);
-
A. Error
-
B. NULLs not allowed
-
C. Both rows inserted
-
D. Only one inserted
Both rows inserted
Both rows inserted
3.What does the UNIQUE constraint ensure?
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A. Values are in order
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B. No NULL values
-
C. No duplicate values
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D. Only one column in the table
No duplicate values
No duplicate values
4.Can a table have multiple UNIQUE constraints?
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A. No
-
B. Yes
Yes
Yes
5.Which of the following allows NULL values?
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A. PRIMARY KEY
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B. FOREIGN KEY
-
C. UNIQUE
-
D. CHECK
UNIQUE
UNIQUE