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MCQ | SQL – Keys

SQL Keys MCQ: This section contains the Multiple-Choice Questions & Answers on SQL Keys.
Submitted by Anushree Goswami, on September 28, 2021

SQL Keys MCQs

1. A Key which uniquely identifies each row in the table is known as?

1. Primary Key
2. Unique Key
3. Composite Key
4. Foreign Key

Answer: A) Primary Key

Explanation:

A Key which uniquely identifies each row in the table is known as Primary Key.

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2. A Primary Key is basically a –

1. Row
2. Column
3. Table
4. Database

Answer: B) Column

Explanation:

A Primary Key is basically a Column or Columns.

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3. If multiple columns are used as Primary Key, it is known as –

1. Unique
2. Composite
3. Foreign
4. None of the above

Answer: B) Composite

Explanation:

If multiple columns are used as Primary Key, it is known as Composite Key.

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4. Which of the following statement is TRUE about Primary Key?

1. Table integrity is not enforced by the primary key.
2. The data in a primary key is always multiple.
3. 900 bytes is the maximum length of a primary key.
4. Null values are allowed in primary keys.

Answer: C) 900 bytes is the maximum length of a primary key

Explanation:

we can have up to 16 columns as primary key column and the total size of the key columns should be less than or equal to 900 bytes.

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5. Primary keys can be contained in a table only ____?

1. Once
2. Twice
3. Thrice
4. None of the above

Answer: A) Once

Explanation:

Primary keys can be contained in a table only once.

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6. To add a Primary Key constraint after table is created, which clause is used?

1. UPDATE
2. ADD
3. ALTER
4. JOIN

Answer: C) ALTER

Explanation:

To add a Primary key constraint after table is created, ALTER clause is used.

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7. In order to remove a primary key constraint, which clause is used?

1. DELETE
2. DROP
3. ALTER
4. REMOVE

Answer: B) DROP

Explanation:

In order to remove a primary key constraint, DROP clause is used.

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8. In order to build a link between two tables, which key is used?

1. Primary
2. Foreign
3. Composite
4. Unique

Answer: B) Foreign

Explanation:

In order to build a link between two tables, foreign key is used.

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9. Which of the following statement is FALSE?

1. A Primary Key cannot be NULL
2. A Foreign Key cannot be NULL
3. A Primary Key cannot be Duplicate
4. A Foreign Key can be Duplicate

Answer: B) A Foreign Key cannot be NULL

Explanation:

The option (B) is false, because a foreign key can be NULL or duplicate.

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10. Foreign key is a field in a table that is _______ key in another table?

1. Primary
2. Unique
3. Composite
4. None of the above

Answer: A) Primary

Explanation:

Foreign key is a field in a table that is Primary Key in another table.

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11. When two or more columns are combined to be used to uniquely identify each row in the table, it is known as -

1. Primary Key
2. Unique Key
3. Composite Key
4. Foreign Key

Answer: C) Composite Key

Explanation:

When two or more columns are combined to be used to uniquely identify each row in the table, it is known as Composite Key.

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12. For one table, there can be ____unique key constraint(s).

1. 1
2. NULL
3. Many
4. None of the above

Answer: C) Many

Explanation:

For one table, there can be many unique key constraints.

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13. All other columns other than the column which is termed as Primary Key, are known as –

1. Unique Keys
2. Alternate Keys
3. Composite Keys
4. None of the above

Answer: B) Alternate Keys

Explanation:

All other columns other than the column which is termed as Primary Key, are known as Alternate Keys.

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