In SQL Server, Set operations allow the results
of multiple queries to be combined into a single result set. Set operators
include UNION, UNION ALL, INTERSECT, and EXCEPT. Before using the set
operations in SQL Server, we should keep the following things in our mind -
1. These set operations are design to work
around more than two datasets or queries
2. Each dataset or query must have the same
number of columns
3. Each column must have compatible data types,
4. Column names for the final result set are taken
from the first query or datasets
Union - This operator is design to merge more
than two datasets and it eliminates duplicate rows if any. It gives unique
record from all data set.
Union all - This operator to design merge more
than two datasets and does not eliminate duplicate rows if any. It gives
all records from all data set.
Intersect - It will show when rows are identical
and not include rows which are different. It will remove duplicate rows from
the final result set.
EXCEPT - It takes the distinct rows of one query
and returns the rows that do not appear in a second result set. The EXCEPT ALL
operator does not remove duplicates. For purposes of row elimination and
duplicate removal, the EXCEPT operator does not distinguish between NULLs.
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