Online Coding Lessons SQL and Database Basics 3

Question 1

Which SQL command permanently removes an entire table and all its data from the database?

Accepted Answer

DROP TABLE

Answer

DROP TABLE permanently deletes the table structure and all its data from the database, unlike DELETE which only removes rows.

Question 2

What does a foreign key in a database table do?

Accepted Answer

Links a column in one table to the primary key of another table

Answer

A foreign key establishes a referential relationship between two tables by pointing to the primary key in another (referenced) table.

Question 3

Which SQL aggregate function calculates the average value of a numeric column?

Accepted Answer

AVG()

Answer

AVG() is the SQL aggregate function that computes the arithmetic mean of all non-NULL values in a specified numeric column.

Question 4

What is the correct SQL syntax to select all columns from a table named 'students'?

Accepted Answer

SELECT * FROM students

Answer

SELECT * FROM students is the correct syntax — the asterisk (*) is a wildcard that means 'all columns.'

Question 5

In SQL, what does NULL represent?

Accepted Answer

The absence of any value

Answer

NULL in SQL represents the absence of a value — it means no data was provided for that field, which is different from zero or an empty string.

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