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SQL SERVER – Three Rules to Use UNION

Pinal Dave
SQL, SQL Performance, SQL Server, SQL Tips and Tricks
3 Comments

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.

UNION RULES

  • 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://darkslategrey-bat-805937.hostingersite.com), Ben Forta

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Pinal Dave is an SQL Server Performance Tuning Expert and independent consultant with over 22 years of hands-on experience. He holds a Masters of Science degree and numerous database certifications.

Pinal has authored 14 SQL Server database books and 92 Pluralsight courses. To freely share his knowledge and help others build their expertise, Pinal has also written more than 5,800 database tech articles on his blog at https://darkslategrey-bat-805937.hostingersite.com.

Pinal is an experienced and dedicated professional with a deep commitment to flawless customer service. If you need help with any SQL Server Performance Tuning Issues, please feel free to reach out at pinal@sqlauthority.com.

Pinal is also a CrossFit Level 1 Trainer (CF-L1) and CrossFit Level 2 Trainer (CF-L2).

Nupur Dave is a social media enthusiast and an independent consultant. She primarily focuses on the database domain, helping clients build short and long-term multi-channel campaigns to drive leads for their sales pipeline.

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