UNION ALL - SQL

Further varieties of UNION arise when we replace the key word DISTINCT by ALL in any of the foregoing examples, as in Example. ALL specifies that if row r appears n times in one operand and m times in the other, then it appears n+m times in the result-i.e., no elimination of duplicate rows takes place.

Example: UNION ALL

SELECT StudentId

FROM IS_CALLED

WHERE Name = 'Devinder'

UNION ALL

SELECT StudentId

FROM IS_ENROLLED_ON

WHERE CourseId = 'C1'

Clearly, UNION ALL represents another departure from relational theory. However, it is commonly used when the operands can be guaranteed to be disjoint because in such cases omission of ALL would incur the possibly significant overhead of the duplicate elimination process with no effect on the final result. Some authorities have argued that there really ought to be yet another variety of UNION, such that if row r appears n times in one operand and m times in the other, with m, n, then it appears m times in the result. Relational devotees might smile at this observation but refrain from comment.