SQL SERVER – Three Rules to Use UNION

I have previously written two articles on UNION and they are quite popular. I was reading SQL book Sams Teach Yourself Microsoft SQL Server T-SQL in 10 Minutes By Ben Forta and I came across three rules of UNION and I felt like mentioning them here.


  • A UNION must be composed of two or more SELECT statements, each separated by the keyword UNION.
  • Each query in a UNION must contain the same columns, expressions, or aggregate functions, and they must be listed in the same order.
  • Column datatypes must be compatible: They need not be the same exact same type, but they must be of a type that SQL Server can implicitly convert.

SQL SERVER – Union vs. Union All – Which is better for performance?
SQL SERVER – Insert Multiple Records Using One Insert Statement – Use of UNION ALL

Reference : Pinal Dave (https://blog.sqlauthority.com), Ben Forta

SQL Joins, SQL Scripts, SQL Union clause
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3 Comments. Leave new

  • Jwalant Natvarlal Soneji
    October 14, 2007 7:52 pm

    But if union and then group by is good or directly using inner or outer join is better?

  • Suppose, I don’t have same number of parameters for second table. I want one field which is not present in second table. But, I want to keep that column. It should show null values for second table which doesn’t have that field and should show data for the table which has that field.


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