What does union mean in sql?

UNION is an operator in SQL used to merge the result sets of two or more SELECT statements. It returns a new result set containing all unique rows. UNION only applies to SELECT statements with the same structure (consistent number of columns and data types) and automatically eliminates duplicate rows.

The meaning of UNION in SQL

UNION is an operator in SQL that is used to combine two or more The result set of multiple SELECT statements with similar results. It combines all unique rows from the two result sets into a new result set.

Detailed description

• Function: UNION is used to combine the results of multiple SELECT statements and return a new result set. Contains all unique rows.

• Syntax:

  SELECT ...
  FROM ...
  [UNION]
  SELECT ...
  FROM ...

• Result set: The result set of UNION contains two (or more) SELECT statements All unique rows in the result. The number of columns and data types of the result set must be consistent.
• Duplicate elimination: The UNION operator automatically eliminates duplicate rows, which means that duplicate data will not appear in the result set.
• Note: The UNION operator is only applicable to SELECT statements with the same structure (consistent number of columns and data types).

• Example:

  SELECT name, age
  FROM students
  UNION
  SELECT name, age
  FROM teachers;

  This example will combine all unique # in the students and teachers tables The ##name and age columns are combined into a new result set.

The above is the detailed content of What does union mean in sql?. For more information, please follow other related articles on the PHP Chinese website!