Compound Query Operators
The compound query operators vary among database vendors. The ANSI standard includes the UNION, UNION ALL, EXCEPT, and INTERSECT operators, all of which are discussed in the following sections.
The UNION Operator
The UNION operator is used to combine the results of two or more SELECT statements without returning any duplicate rows. In other words, if a row of output exists in the results of one query, the same row is not returned, even though it exists in the second query that combined with a UNION operator. To use UNION, each SELECT must have the same number of columns selected, the same number of column expressions, the same data type, and have them in the same order—but they do not have to be the same length.
The ...
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