DA SQL and Database Querying 1 — Questions and Answers
Question 1: Which SQL clause is used to filter rows after a GROUP BY operation?
- WHERE
- HAVING (Correct answer)
- ORDER BY
- FILTER
Correct answer: HAVING
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?
- Only matching rows from both tables
- All rows from the left table and matching rows from the right (Correct answer)
- Only rows unique to the left table
- All rows from both tables with no filtering
Correct answer: All rows from the left table and matching rows from the right
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?
- COUNT(*)
- SUM()
- COUNT(column_name) (Correct answer)
- AVG()
Correct answer: COUNT(column_name)
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?
- Sort results in ascending order
- Remove duplicate rows from the result set (Correct answer)
- Filter rows based on a condition
- Join two tables together
Correct answer: Remove duplicate rows from the result set
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?
- INSERT
- ALTER
- UPDATE (Correct answer)
- MODIFY
Correct answer: UPDATE
UPDATE changes existing records in a table based on a specified condition in the WHERE clause.
Question 6: What does the SQL ORDER BY clause do?
- Groups rows by a column value
- Sorts result rows by one or more columns (Correct answer)
- Filters rows meeting a condition
- Removes duplicate values
Correct answer: Sorts result rows by one or more columns
ORDER BY sorts query results in ascending (ASC, default) or descending (DESC) order based on specified columns.
DA SQL and Database Querying 1
Which SQL clause is used to filter rows after a GROUP BY operation?