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GeekInterview.com > Interview Questions > Oracle > SQL
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| Question: What is difference between UNIQUE and PRIMARY KEY constraints?
Answer:
A table can have only one PRIMARY KEY whereas there can be any number of UNIQUE keys. The columns that compose PK are automatically define NOT NULL, whereas a column that compose a UNIQUE is not automatically defined to be mandatory must also specify the column is NOT NULL. |
| July 07, 2007 05:20:48 |
#6 |
| Faizal |
Member Since: Visitor Total Comments: N/A |
RE: What is difference between UNIQUE and PRIMARY KEY ...
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/*
Just adding to the Primary Key Explanation
*/
Its right that there can be only one Primary Key for a Table.
But for that Primary Key we can add more than one fields.
Ex:
ALTER TABLE emp ADD CONSTRAINT PRIMARY KEY(emp_no, dept_no, phone);
By the above, all the 3 fields make a combination of a Primary Key.
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