What does UNION do?

What does UNION do?

• A. UNION of two SELECT results returns only distinct rows that appear in both results.
• B. UNION of two SELECT results concatenates the columns of the two results side by side.
• C. UNION of two SELECT results requires the same number and compatible data types in the columns.
• D. UNION combines the results of two SELECT statements by listing all rows from the first and all rows from the second.

Answer: (D)

UNION combines the results of two SELECT queries into a single result set and removes any duplicate rows, giving you the union of the two results. This means a row that appears in either query will appear once in the final output, but only if it matches across all the selected columns. To use UNION, the two SELECT statements must return the same number of columns and the corresponding columns must have compatible data types. Otherwise the database system can’t align the results row by row. If you want to keep duplicates and simply attach all rows from the second query after the first, you’d use UNION ALL. That differs from UNION, which de-duplicates. Also, the column names in the result typically come from the first SELECT (or its aliases) unless you explicitly alias them in the outer query.

UNION combines the results of two SELECT queries into a single result set and removes any duplicate rows, giving you the union of the two results. This means a row that appears in either query will appear once in the final output, but only if it matches across all the selected columns.

To use UNION, the two SELECT statements must return the same number of columns and the corresponding columns must have compatible data types. Otherwise the database system can’t align the results row by row.

If you want to keep duplicates and simply attach all rows from the second query after the first, you’d use UNION ALL. That differs from UNION, which de-duplicates.

Also, the column names in the result typically come from the first SELECT (or its aliases) unless you explicitly alias them in the outer query.