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The Union operator can combine a collection of query results from two or more SELECT statements into a single result set, that is, to execute a federated query. The syntax format of the Union is:
UNION [All] selectstatement
[UNION [All] selectstatement] [... n]
Where Selectstatement is the SELECT query statement to be joined.
The all option means that all rows are merged into the result collection. When this item is not specified, the duplicate rows in the Federated query result collection are persisted to only one row.
In a federated query, the column headings of the query results are the column headings of the first query statement. Therefore, to define a column heading must be defined in the first query statement. To sort federated query results, you must also use the column name, column heading, or column ordinal in the first query statement.
When using the union operator, you should ensure that there are the same number of expressions in the select list for each federated query statement, and that each query selection expression should have the same data type, or you can automatically convert them to the same data type. In the case of automatic conversion, the system converts a low precision data type to a high precision data type for numeric types.
In a union statement that includes multiple queries, the order of execution is from left to right, and parentheses can be used to change the order of execution. For example:
Query 1 Union (query 2 union query 3)
Original Author: unknown
Source: Design Practice of MSSQL
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