DA SQL and Database Querying 1

Question 1

Which SQL clause is used to filter rows after a GROUP BY operation?

Accepted Answer

HAVING

Answer

HAVING is used to filter aggregated results produced by GROUP BY, whereas WHERE filters individual rows before aggregation.

Question 2

What does a LEFT JOIN return?

Accepted Answer

All rows from the left table and matching rows from the right

Answer

A LEFT JOIN returns all rows from the left table and the matched rows from the right table; unmatched right-table rows appear as NULL.

Question 3

Which aggregate function counts only non-NULL values in a column?

Accepted Answer

COUNT(column_name)

Answer

COUNT(column_name) counts only non-NULL entries in that column, while COUNT(*) counts all rows including those with NULLs.

Question 4

What is the purpose of the SQL DISTINCT keyword?

Accepted Answer

Remove duplicate rows from the result set

Answer

DISTINCT eliminates duplicate rows from query results, returning only unique value combinations.

Question 5

Which SQL statement is used to modify existing data in a table?

Accepted Answer

UPDATE

Answer

UPDATE changes existing records in a table based on a specified condition in the WHERE clause.

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