MS SQL Basics Tutorial: Merging queries

A merge query uses the union operator to combine data from different queries to form a query result with comprehensive information. The UNION operation automatically excludes duplicate rows of data. It is important to note that each subquery participating in a merged query uses the same table structure, that is, the number of data in each subquery and the corresponding data type must be the same.

In a SELECT statement that uses union, it is best to specify a sort order in a sort clause by using numbers, and if you do not use numbers, the names of the columns in the subquery for the merged query need to be the same. You can use aliases to unify column names.

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