What is the difference between UNION and UNION ALL?

by

This deals with SQL. UNION only selects distinct values, UNION ALL selects all values.

In the context of Core Java, the terms UNION and UNION ALL are not directly related. Instead, they are typically associated with SQL (Structured Query Language), which is used for database operations.

In SQL:

  1. UNION:
    • The UNION operator is used to combine the result sets of two or more SELECT statements.
    • It removes duplicate rows from the combined result set.
    • The data types in the corresponding columns of the SELECT statements must be compatible.

    Example:

    sql
    SELECT column1, column2 FROM table1
    UNION
    SELECT column1, column2 FROM table2;
  2. UNION ALL:
    • The UNION ALL operator is also used to combine the result sets of two or more SELECT statements.
    • Unlike UNION, it does not remove duplicate rows from the combined result set. It includes all rows, even if they are duplicates.
    • The data types in the corresponding columns of the SELECT statements must still be compatible.

    Example:

    sql
    SELECT column1, column2 FROM table1
    UNION ALL
    SELECT column1, column2 FROM table2;

So, in summary, the main difference between UNION and UNION ALL is that UNION removes duplicates, while UNION ALL includes all rows, even if they are duplicates. This concept is not specific to Core Java but is applicable to SQL when working with databases.