MySQL UNION

UNION puts the results of two or more SELECT queries one below the other.

• Every query must have the same number of columns.
• The columns must have compatible data types, in the same order.
• The column names of the first query are used in the result.

SELECT city AS name FROM students
UNION
SELECT course_name FROM courses;

UNION removes duplicate rows. Delhi and Mumbai appear twice in the students table, but only once here.

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UNION ALL

UNION ALL keeps the duplicates, and it is faster.

SELECT city FROM students
UNION ALL
SELECT course_name FROM courses;

This returns 10 rows: 6 cities (with the repeated Delhi and Mumbai) and 4 courses.

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A practical example

SELECT name, 'Top' AS level FROM students WHERE marks >= 85
UNION
SELECT name, 'Low' AS level FROM students WHERE marks < 60;

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Summary

• UNION combines results and removes duplicates, UNION ALL keeps them.
• All queries need the same number of columns.
• Next: Indexes.

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